hrm... lessee lesseeee... i would have to break this one down and analyze some of my favorite aspects...
for one i really really liked the treasure mechanic in squeak squad, getting things to mess around with, including little cosmetics for kirby, was so fun... even could probably mix it with the ability evolution mechanic from katfl... being able to customize the appearance of abilities to some degree without having to stick to the aesthetic? i love collectables, i love customization, i'd go crazy for it... i also do love when there's multiple player characters... not necessarily to the extent of star allies but i would love to see more on the scale of return to dreamland's multiplayer!! i think the most fun postgame modes as well are the ones like dededetour and meta knightmare ultra, especially when they have some distinct story divergence....
as for story i am most drawn to the very heavy interpersonal ones... i suppose having multiple major players in the backstory gives me more to chew on... added layer of complexity when i get to think about the motivations not just of One Guy like magolor but like, the struggle of multiple people overlapping!!
would also... really love to see them go somewhere with the art sisters. but that seems unlikely between drawcia being a spinoff villain and neither of her sisters being very important to any specific plot...

OH! i also really enjoy like... waddle dee town, conceptually, i think it would be cute to see hubworld towns more in (inevitable, they wouldn't make this whole new engine and not make more) future 3d installments.... and uhm my sole gripe with katfl is just that by nature of being 3d we lost a lot of the combo depth but. i would not know how to resolve that and still have it feel good! so i will live with it