Venser, the Sojourner is my favorite card, because it speaks to me from multiple angles.
First and foremost, I love blink effects. Something as small as a blink can change the way a lot of things operate, by repeating enter-the-battlefield triggers, rescuing your mind-controlled or perpetually-tapped creature or other permanent, or even just giving something pseudo-vigilance. Or, perhaps in a cube, you can do something nasty like blink a wincon and proceed with a wrath - or even nastier, an Upheaval - and continue onward uncontested. It's a lot of versatility, making Venser's first ability elegant, even though it's incredibly simple.
Venser's second ability is perhaps a bit confusing to a control deck trying to abuse his +2 with the likes of Wall of Omens or Mulldrifter; why would you ever use this ability? I always believed this is because Venser was designed for the BantPod deck when it was standard legal, a midrange monstrosity that abused etb triggers and eventually amassing a huge board. With Venser, you can build up advantages until you can simply overwhelm your opponent; a -1 with "Creatures are unblockable this turn" could just as well read "win the game" in the right deck. I enjoy cube, and find Venser doesn't need to use this ability to be good, but having the option allows him to fit in more decks than many other planeswalkers who don't have as straightforward abilities as "Draw a bunch of Cards". For this reason I think he trumps most other guys outside the Lorwyn 5. Both the +2 and -1 can work together, but they don't need to.
Venser's ultimate is incredibly powerful - a little wonky, to be sure, but powerful. Spells giving you free, colorless exiles is exceptionally good, and cantrips like Preordain suddenly become incredibly valuable. If you have a Batterskull on the table, you're in amazing shape. And this isn't an ultimate that takes forever to get to, either.
Venser is far from the best card, or even best planeswalker in MTG. But, all three of his abilities are potentially powerful, which lead to enjoyable experiences every time I play it. Because Venser's abilities require a little bit of planning, I find that deck construction with him becomes quite interesting; some already good cards gain additional value, some strategies suddenly have an entirely new angle when he hits the field. Being able to think ahead and see real benefits of that planning make playing the card a very rewarding experience.
1GG - Sorcery
You may sacrifice a nontoken green creature rather than pay this spell's mana cost.
Search your library for up to two basic land cards, reveal those cards, put one onto the battlefield tapped and the other into your hand, then shuffle.
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Any interest in this card?
I still include a copy of Cultivate in my cube, and although the two-pip cost might be a little more narrow, I think green can tolerate it for the access to the alternate cost. But it doesn't answer the most pressing question: why would we want to sacrifice a nontoken creature instead of paying the mana cost, anyway? It certainly doesn't seem intuitive to sac your Llanowar Elves, nor do I really want to pitch a Tireless Tracker or a Tarmogoyf. So what is this clause even for?
I think there's a case for certain etb- / death- trigger creatures in green that might give this card some explosive power. Although not really popular these days, my first consideration was for cards like Wall of Blossoms and Sylvan Ranger, and then maybe the less popular Wood Elves ... but I then realized that a local favorite, Caldaia Guardian, makes for prime fodder. I think another potentially clever use could be dropping an Esika's Chariot, animating it immediately for this sacrifice, still leaving you with two tokens (albeit tapped).
My fear is that most cubes will lack the concentration of good sac targets in green, making this just a slightly worse Cultivate in the average case scenario. So, if you've already cut Cultivate, this probably isn't a good include. But I'm nevertheless curious if this has anyone else's gears turning, and if they have any other thoughts I've neglected to add here.
I think the non-face burn is absolutely worth considering, but I don't have any desire to cut the face-targeting cards like Lightning Strike. I'm currently trying to include a variety of these spells so that it's not so one-note (and you don't get screwed as a red player the moment you face something with 4 toughness).
While I absolutely encourage folks to look for interesting synergies in their lists, I think also having general-use cards like this supports a really fun cube environment.
I don't have any immediate plans to add it, but I'll be curious to know if other folks here really enjoy it.
So basically I agree, it's not for every list, but if you support a black midrange deck that performs decently well, this is probably a fun addition.
I feel like this could be a nice aspect of the card. Works with Seasoned Pyromancer, Fable of the Mirror-Breaker, and at least somewhat recently I know people liked Bitter Reunion. To me, that's solid value that I can get by just running other good cards.
The first ability I think people might not want to use too aggressively. Going down a card is costly and can be an actual risk if for some reason your impulse draw doesn't work out. But, if I can spend a card to get a creature outside of range of trading with my opponent's blockers, I'd happily pay that price.
Overall I think it's intriguing enough to give a shot.
I'm still playing krasis though