I generally just cut oketra since it always feels like the worst card of the deck, maybe I'll just replace it with tosmir.
I kinda want to fit one more self bounce but all the ones that are creatures suck.

I love how the meta is far from settled. I think WAR cards are gonna continue shaping it for the foreseeable future (methinks Wizards printed an OP set to put a sort of reset button on the meta of ixalan-ravnica). I feel like there are lots of broken interactions that have yet to be exploited. It will be really interesting to see what archetypes emerge as dominant.

But yeah, re: Oketra, I've been severely underwhelmed. I almost never get the zombie tokens, and they don't have flying, which has been the achilles heel in the bant midrange deck I'm using. Oketra should really have reach, or, idk, vigilance, or those zombie tokens should have flying.

I think you might be underestimating Oketra somewhat. If she lives, she will generate effectively-unbeatable value. If she doesn't live, she'll still probably return to play in 3 turns at most. I find her quite a scary card. The zombie tokens not having flying isn't too bad, since I find that not that many games are won by races in the air. Besides if you cut her what are you going to run? In the 5-drop slot you're looking for creatures that outright win the game if unanswered, and there aren't many competitors to Oketra - maybe Lyra if you are behind (I sideboard that in against monored) or Trostani (has an ETB, but not nearly as devastating if left unanswered).

The main concern with Oketra is if I play her, she dies to some cheap removal (or even worse gets answered for no mana by an already-in-play Teferi), and I promptly get overrun. That's mitigate-able to some extent by holding on to cheap creatures and waiting till 6+ mana, e.g. Oketra + Llanowar Elves will immediately make a 4/4. It's hard to see Tolsimir as comparable to Oketra in terms of power level - against some decks the life + removal will matter (e.g. monored, although Chainwhirler attacks freely into Tolsimir), against others it's just an inferior creature (e.g. Bant - Tolsimir is irrelevant against an Oketra on the other side of the board).

Maybe it just doesn't fit in my deck, which is kinda a janky hybrid of simic nexus (minus the nexus) and bant midrange. Oketra's been good when I'm casting it on my opponent's end step using vivien (and usually wilderness reclamation in order to have untapped mana). It's rarely been that effective when cast during my main phase. But in that situation the better play is hydroid krasis, or some other finisher. Idk I'll have to keep on testing. When I was running one copy she seemed pretty good, running two though I felt like I kept drawing her at unwanted points.

I dunno, I know that the whole deck basically revolves around vivien, but having a wincon that requires vivien to be above average feels a bit too cute.

Well as long as you are running Bant, what about Eternal Skylord?
A little costly for a competitive standard deck but gives your wishes.

Update: Okie definitely won me some games last night. Sailed into platinum, where the competition is decidedly tougher. Will this be the season when I hit mythic?

The meta is so ... I don't know how to put it ...

I'd ranked up twice easily with Esper hero, and then ran into Esper superfriends with Cry of the Carnarium + Kaya's Wrath, a B/G land destruction deck with 4 Field of Ruin and complete with Crucible of Worlds, and 4c Command the Dreadhorde (did anyone even teach these guys about greedy mana bases?). All of them squash Esper hero. My clock just isn't very fast, my value creatures aren't that valuable, and Narset is almost as opporessive as Teferi.

In the end I'm playing monored ... after all no matter what fancy plan opponent is trying to execute, you can always trump that by just killing them. Sometime in the future I think I will still try to put together a The Immortal Sun deck, simply because the mass of planeswalkers is just asking to be attacked in this way.

Simic Mass Manipulation is fun! It's not that straightforward to play (I lost count of how many times I animated the wrong land with Nissa in my first few leagues with the deck), but I'm rapidly getting better, and I've scored my first 5-win league too. I know I said the same about Bant earlier, but I think I'm going to take this up as my main deck this season

What league ?

Traditional constructed ... haven't played any ranked BO1 or league for a long time.

Anyone know how the Jeskai planeswalkers vs. Esper planeswalkers matchup is like? It seems like a pretty silly matchup, especially from the Esper side, since neither side can effectively kill the other's planeswalkers (Jeskai at least has Sarkahn).

Black has the elder spell...

Yeah, but how many copies of that card are people running mainboard?