Dont forget to include other lifegain payoffs, like Speaker of the Heavens, and ways to get even more +1/+1 counters, like Conclave Mentor
Some spells with amass require a different target
Intrepid Adversary also just scales more easily with the ability being paid in two-mana increments as opposed to three
Whitemane Lion and Jeskai Barricade cost just two mana, and you can flash them in while Oketra is on the field to create 4/4 zombies, then return them to hand to repeat ad nauseum
So much so, in fact, you may want to pair it with similar cards such as Dryad of the Ilysian Grove
One could argue even Gamma playtest cards implied a Creature type in that the Creatures themselves were of various types (e.g., Drudge Skeletons and Shivan Dragon)