what do you think the next big pokemon abilaty will be (sorry about the poor spelling

Fusion would be too complicated for them to want to add. Also, I have a feeling they want to stay away from things that people might feel don't add much, like D/Gmax and Z-moves (and though I think the latter is great, most people don't agree). With Terastallization, they aimed for competitive use that'll make people use it more and strategise around it, and in Z-A, they brought back Megas, a feature everyone wanted back because it gave them cool designs and power boosts that felt more genuine and less like the 1 or 3 turns of strength that D/Gmax and Z-moves were.

I think at the very least, it'll be something people will love, because TPC and GameFreak have been obsessed with trying to make things like that lately (I think 95% of Z-A is just them trying to appeal to as many people as they can without making the games actually better, resulting in a game that's not very good. It's funny considering XY was like a predecessor to that, with its immense amount of gen 1 references that is impossible to exaggerate).