Ire Shaman way over Ash Zealot. These days, a permanent has to be flat awesome to justify CC mana costs. I've got a few: Bloodghast and Rofellos. That's it. Anything else isn't worth the hassle of having to run 13 sources or whatever to reliably stick it on T2.
Unless you include CC cards as a design choice. I actually like to encourage decks that are heavily in one color and just splash for a second instead of having most decks being solidly in 2+ colors.
Kargan Dragonlord: At a quick glance your Cube (like mine) does not play fast mana, and I really like it in this context.
Of the three cards you suggest, I'd probably add Lightning Mauler and Hellspark Elemental. I am not a fan of "player burn creatures" in general, but Hellspark does have good synergies (not only with sacrifice outlets but also with looters).
I see all the tools for fast reanimator. I think Exhume would be quite good in your list, even though you don't have very many big monsters to get back from the graveyard (you don't want too many, so maybe the number is exactly right).
I feel that both Erratic Portal and Crystal Shard suffer from the ever increasing tempo in Cube decks, at least for smaller lists. But I agree with wtwlf123, I'd probably add neither or both.
I prefer talismans, because you are not forced to use mana in chunks of two. I would run talismans where possible.
With enough signets/talismans, it is often almost as easy to splash the fourth or even fifth color as it is to splash the third. I prefer the 4/5-color good-stuff decks to require some effort regarding the mana base rather than it coming together incidentally. Therefore, I prioritize non-signet/talisman mana rocks and fill up with signets/talismans as needed. Coldsteel Heart, Everflowing Chalice, Guardian Idol, Mind Stone and Prismatic Lens are all reasonable choices. Fellwar Stone, Star Compass and Sphere of the Suns are not as good as signets but still have the "problem" of incidental color fixing, so I choose not to run them. Thought Vessel and Fractured Powerstone virtually or actually don't do anything other than tapping for 1; they are options if you want to further reduce the color fixing available on mana rocks.
That is what we thought until now, but we do Rochester full table and support Sneak Attack, Eureka and big ramp. This is what made me doubt. The Wurm is also big enough to make it worth to try and combo it out.
Even in a booster draft scenario I found the super-fatties to be in high demand as soon as you add enough ways to cheat them into play. Worldspine Wurm and Progenitus have the additional upside of being green if you support Natural Order. The Wurm also leaves some bodies behind when used with Sneak Attack or Through the Breach. I don't know how much your players like Terastodon, but I'd give the Wurm a try for sure!
I suggest running all the humans from your list and Isamaru, Hound of Konda. Running so many similar cards may seem boring, but one drops are very important for aggro decks. Make sure to check your creatures at 2-4 mana: it is a relatively new phenomenon that we have a decent number of one drops, and before that we used to make up for the lack of one drops with more expensive creatures (two drops, mostly). Now that we have the one drops, some of those more expensive creatures have to go imo in order to make room for the one drops.
At least for Cubes with 450 cards or fewer, not playing all the swords is worth considering imho. The swords cost 5 to play and equip, which is quite pricey for aggro decks, but they require a healthy creature count, which is something control decks often don't have. This, the raw power of the swords and their colorless nature makes it so that running all five swords in small or medium sized Cubes provides an over-proportional incentive to play midrange decks. Cutting one, two or even three of the five swords and replacing them with colorless aggro and control options may lead to a better balance of the Cube.
Falkenrath Aristokrat has a great power to mana cost ratio for an evasive haste creature. In addition to that, the ability to sacrifice creatures at will is a powerful one, and you even get a powerful effect when you do it! I think it performs really well in many flavors of Rakdos decks. It is an absolutely top tier Rakdos card.
I like the green cards that interact with creatures, especially Survival of the Fittest and Birthing Pod. The recursion on Vengevine is more than flavor text in those decks. It also plays well with or against Upheaval, and Wraths are much easier to beat when you have Vengevine beating down for 4 a turn and 2 creatures in hand. It is a cutable card, for sure, but I think it has fun interactions.
I play Horizon Canopy in addition to the four lands per guild I run. Traditional dual lands are like gold cards: if you can't use both colors, the land doesn't do anything for you. Horizon Canopy is like a hybrid card: it is perfectly playable if you play white OR green, and it is at its best if you play both.
I'd love a Horizon Canopy cycle. It would make me increase the number of lands per guild from 4 to 5. Lands that tap for two different colors turn 1 and that can be cashed in for a card later are great for any deck. The life loss is more of a concern for control decks than for aggressive decks, which is great because it balances the Worldwake manlands (which are also great for any deck, but better for slower decks than for faster decks that really want their turn one mana).
How big a Cube are we talking here? Sunblast Angel only kills tapped creatures, so even though it is one more mana, I'd prefer Martial Coup. Akroma's Vengeance does kill artifacts which may hurt you more than your opponent (artifact mana), but does not kill planeswalkers (which may or may not be good for you, depending on the board state). Planar Cleansing would be a true reset button, but a mix of the 4 and 5 mana wraths to target creatures and flexible Oblivion Ring-style removal for everything else seems more desirable to me.
Xathrid Necromancer is easily the worst card on that list. The human count is growing with each new set, but we're not close enough on the amount of good ones to make cards like this one good.
I like Lotus more than Dynamo, since Lotus can effectively cost 2 a reasonable amount of the time by allowing you to play a coloured spell on the same turn. Playing something significant with 3 compared to WWW, say, is much trickier.
This is a good point in general. One of my design goals is to encourage decks that are 1.5-3 colors, so I'm playing Dynamo over Lotus.
Also, I'm not sure where you guys get the stats on double mana of a given color by turn 4, but I haven't had problems with this. Maybe I'm just really lucky.
"What am I looking at? Ashes, dead man."
Mogg War Marshal is a good token card.
Kargan Dragonlord: At a quick glance your Cube (like mine) does not play fast mana, and I really like it in this context.
Of the three cards you suggest, I'd probably add Lightning Mauler and Hellspark Elemental. I am not a fan of "player burn creatures" in general, but Hellspark does have good synergies (not only with sacrifice outlets but also with looters).
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
With enough signets/talismans, it is often almost as easy to splash the fourth or even fifth color as it is to splash the third. I prefer the 4/5-color good-stuff decks to require some effort regarding the mana base rather than it coming together incidentally. Therefore, I prioritize non-signet/talisman mana rocks and fill up with signets/talismans as needed. Coldsteel Heart, Everflowing Chalice, Guardian Idol, Mind Stone and Prismatic Lens are all reasonable choices. Fellwar Stone, Star Compass and Sphere of the Suns are not as good as signets but still have the "problem" of incidental color fixing, so I choose not to run them. Thought Vessel and Fractured Powerstone virtually or actually don't do anything other than tapping for 1; they are options if you want to further reduce the color fixing available on mana rocks.
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
At least for Cubes with 450 cards or fewer, not playing all the swords is worth considering imho. The swords cost 5 to play and equip, which is quite pricey for aggro decks, but they require a healthy creature count, which is something control decks often don't have. This, the raw power of the swords and their colorless nature makes it so that running all five swords in small or medium sized Cubes provides an over-proportional incentive to play midrange decks. Cutting one, two or even three of the five swords and replacing them with colorless aggro and control options may lead to a better balance of the Cube.
"What am I looking at? Ashes, dead man."
I like the green cards that interact with creatures, especially Survival of the Fittest and Birthing Pod. The recursion on Vengevine is more than flavor text in those decks. It also plays well with or against Upheaval, and Wraths are much easier to beat when you have Vengevine beating down for 4 a turn and 2 creatures in hand. It is a cutable card, for sure, but I think it has fun interactions.
"What am I looking at? Ashes, dead man."
I'd love a Horizon Canopy cycle. It would make me increase the number of lands per guild from 4 to 5. Lands that tap for two different colors turn 1 and that can be cashed in for a card later are great for any deck. The life loss is more of a concern for control decks than for aggressive decks, which is great because it balances the Worldwake manlands (which are also great for any deck, but better for slower decks than for faster decks that really want their turn one mana).
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."
"What am I looking at? Ashes, dead man."