How are you feeling about Greatmaw? I haven't been feeling that happy with him, but those feels were mostly cultivated when I was still thinking the deck needed to be more aggressive.
Nissa is the most important adition, then Yah and then Kali, I'd say
Greatmaw...weird, isn't it? I have come to the
concussion that he feels a bit dissapointing because we (probably, at least I did) make the mistake of using him as a counter dump...I think he should be played without counters in him, Exemplar is a better garbage can for those... and just use him as a 4/5 with, sometimes, a little room for shenanigans (like putting a counter from a mummy with no enemy targets in him, so to clear another in another critter while making a Snake and/or an Insect in the process, for example)...those stats are good enough in themselves for 3 mana, the 5 thoughness rocks in a grindy deck.
Only if a crucial creature has too many counters I think it deserves to make him weaker. Also, the 2 counters matter a lot for Nests and Hapys.
I added them, and I think my build now is identical to yours (or nearly at least). I went 3-0 on the ladder with it yesterday. Didn't draw Nissa or Kali in any, but Yahenni came up in 2 of the 3 and is a great fit here (getting pretty big in both). I had her and Bontu out with Nest in one, and their wondertwin powers just took over the game. I'd sac a insect token to make Yahenni indestructible and turn on Bontu, then swing and take out blockers swelling Yahenni and decimating opps board without having to cast removal I was holding. With both resistant to removal, there wasn't much opp could do but watch it happen. Another cool thing about Yahenni was being able to target a weenie for -/- counters, then sac the weenie for indestructible before resolution to make extra use of it (if don't have Nest out).
It does feel a lot better being able to do more useful things with Nest spam. I should have picked up on that earlier when I realized how often Bontu was winning games for me by converting token spam into scry/lifeloss/menace-attack.
Still not super stoked about Greatmaw, lol. I won't pull it out yet because the 2 -/- counters is important for Nest, but I am finding if he's one of a couple 3 drops I can play on curve, I keep picking the other options first. I'm sure there will be a game where I drop Dissenter T2, Greatmaw T3 for a 4/5 and zombie token and be happy about it, but hasnt happened yet.