MTG Salvation

Wort, Boggart Auntie
Borborygmos Enraged
Marath, Will of the Wild
Damia, Sage of Stone
Melek, Izzet Paragon
Teysa, Orzhov Scion
Sek'Kuar, Deathkeeper
Gisela, Blade of Goldnight
Geth, Lord of the Vault
Rubinia, Soulsinger

Sigil of the Empty Throne, Sacred Mesa, and Hoofprints of the Stag were the main win cons in mine, along with planeswalkers. I also had Ancestral Mask, which combined with a flyer will usually kill someone quickly.

Overrun effects are fine and all, but with my b/r goblins I've never had trouble closing out a game where the table lets me keep enough goblins on the table to make an overrun dangerous. Between Krenko, Goblin Bombardment, Sharpshooter, lords, Coat of Arms, Shared Animosity, etc, you should always be able to find something to make a pile of goblins into killers. I think Jund goblins could be fun if you want to work with Shattergang bros, but if you aren't planning to build around that general at all, I don't think green adds that much to a normal b/r goblin deck, not enough to justify the mana base change (though Survival would be sweet). If you do want to go Jund and keep a strong goblin theme, it will probably be a primarily b/r deck splashing green for a few powerful cards and maybe some enchantment removal.

I think the main objection to Zur as an enchantment commander is the lack of green in his color identity. Since green is the traditional color for enchantress effects, people are looking for a commander that will let them play that color. I think if Wizards wants to design an enchanment legend, it should be w/g/x, with w/g/u being the most common colors for that strategy in commander.

Personally I was a bit disappointed to see no enchantment legend in this years product, but the fact that they filled archetype commander holes (Nekusar for group slug, Roon for blink) makes me optimistic for future products. I do hope that we will see one that interacts with enchantments in a more interesting way than simply drawing a card when you cast one. Maybe Karn for enchantents? For now, I've dropped the blue from my deck for red and Marath. We'll see how it goes, I lost some fun stuff, but red offers some good options as well and I like changing things up. Marath at least interacts with doubling seaon etc, where Rubinia didn't interact with the rest of my deck in any way.

When I had my thallid deck I used blue instead of white. My deck was deliberately a lower power deck and so Ghave was not really what I was looking for. Objectively he is definitely the highest power option. I used Damia as my commander, and mainly used blue for proliferate effects. Damia was nice because when you are playing with weaker cards like thallids, it can be very easy to get wrathed and simply not be in the game anymore. Damia lets you reload on bad cards after someone kills all your bad cards.

Mercadia's Downfall

If I attack multiple players in the same combat and cast this spell, how is the power boost determined? It seems to make the most sense that each creature gets a +x based on the number of non-basics controlled by the player that creature is attacking, but I can't find any definite answer.

Opponent A has 2 non-basics
Opponent B has 5 non-basics

If I attack each player with 1 or more creatures, do some get +2/+0 and some +5/+0 or is only one of the opponents the defending player for the purpose of the spell and all attackers get a bonus based on that?

I used to have a similar deck, it was a lot of fun. I liked Myr Retriever and Junk Diver as cheap creatures to get counters on Animar and then recur artifacts later. They also help you recover from board wipes. Another all-star is Trading Post, which is particularly good with those two guys. Shimmer Myr is really good as a flash enabler.

Check out Goryo's Vengeance or Footsteps of the Goryo. The templating is right.

Those two only return a card from your own graveyard, the black land says just from a graveyard.

3 player game with my Damia clone deck, a Kaalia deck and a Kemba deck. Kemba is huge, lifelinking and making lots of cats each turn. Kaalia puts Avacyn into play, and then post combat casts Platinum angel. Kemba attacks Kaalia down to negative life and passes to me. I have 2 clones on the field, one is Kemba and the other is Basandra. I cast Ghostly Flicker, turning my clones into Avacyn and Platinum Angel. Then I use Cytoshape to turn Kaalia's Platinum Angel into a cat. Kemba and I then race to see if he can find removal for an indestructable angel before I can find a way to kill him. I win the race by finding Nacatl War-Pride along with a way to copy it.

Gruul Turf is great if you needed to play a cycling land early in the game, since you can pick it back up to cycle later. I'm a big fan of Seer's Sundial in decks without access to blue or black, it definitely helps provide a steady stream of gas. I don't have it in my list for a couple of reasons. Once I hit a comfortable land total in this deck, I often stop consistently making land drops in order to keep some lands in hand to work with, only dropping lands to keep from having to discard eot. The other reason is simply that I use the Sundial in a couple of other decks and for variety's sake have looked to other draw engines for this deck.

Scapeshift - I saw your explanation as to why you excluded it, but I must insist you try it out. It's absolutely insane with avenger of zendikar/rampaging baloths & sets off massive valakut combos.
Gamble - turns on life from the loam regardless of what you discard.
crucible of worlds - seems like a must have inclusion.
sylvan primordial - fetches 3 lands & destroys 3 permanents.
beast within & chaos warp - two all purpose answers. I'm curious as to why they didn't make the cut.

Scapeshift - Generally, I use Valakut as steady board control or a way to gradually bring life totals down without making people panic, so I'm not sure blowing all my triggers at once is the way I want to go. For Avenger and Baloths, if they are in play Scapeshift is just win-more as they are lethal very quickly anyways.
Gamble - My only Gamble is in another deck
Crucible - Already in my list
Sylvan Primordial - I don't like the card at all and choose not to play it
Beast Within/Chaos Warp - I didn't want many 1 for 1 answers, so these didn't make the cut. If I need more removal these are where I'd start
Fault Line - Chosen over Starstorm because you can kill players or planeswalkers with it

Thought I'd share my build of Borborygmos. I have a ton of fun with this deck. Plenty of ramp to get you enough mana that you can start doing powerful things. You have 3 main paths to take, the main being your commander (with Seismic Assault and Land's Edge for redundancy) and all the shenanigans that go with him. Alternate paths are the big bomby creatures or spells, and a slower card advantage route to build up resources with cycling lands, life from the loam, mind's eye etc. I chose not to go the full combo route with Abundance, and decided against Scapeshift (mainly because I don't have a spare Vesuva).

Goblin Bombardment. One of my favorite EDH cards, and seemed good when I was theorycrafting but it never really did anything. I think red is just the wrong color for this effect, at least in my cube as Mortarpod has been consistently good.

So if I only own 1 sol ring, it shouldn't be put in any other decks I own?

Pretty much, yes. I don't have sol rings for all of my decks, and I don't miss it in the decks that don't have one. In some cases, I actively don't want one (or other mana rocks) because of other card choices in the deck. As others have said, forcing yourself to explore other card choices because you don't have enough staples for all of your decks is a good thing. The exception is if your playgroup is so competitive that anything less than completely tuned is not good enough.

I generally don't like proxies, but can tolerate them if they are a card in testing, waiting to be picked up etc. I still try to discourage people from using them. I think that they encourage the attitude that anything less than the best is crap, when the next tier down is still completely playable. In turn, that encourages a lot of staples and decks that all look similar instead of people exploring a wider range of card choices. Forcing yourself to make due with less is a good way to improve your skills as both a deck builder and a player.

Finding options for better recursion of any card. Codex Shredder is an option, but I want a better way of pulling useful stuff out of my Graveyard if it gets loamed.

Regrowth, Restock, Nostalgic Dreams and Recollect are probably the best, in addition to the Witness you already have.