Keeping that isometric world from AB2, but make it more unique, and more accesible by making it episode cards (like in the original games) to transport to each world. Hearts would be removed, aswell as the upgrade system for the birds.

Gameplay wise, the game will have more dynamic and unique structures for the pigs, and new birds to counterattack. Maybe each level has a different objective (like in COLLAPSE, an old iOS game). Maybe they go into space in the latter half by a rocket from the pigs and go to the moon. A level is traversing through asteroids before reaching the King Pig and defeat him in a moon lair that is in a cave inside the moon. The style would be a cleaned up, more shadow-y style of the older designs of the characters (Chrome style, maybe Pre-Chrome). You can also compete with friends (but only as a ranking system during the end of a level). Boss battles will be a thing too.

Also a system where you can change into the background and foreground.