I really despise how they explained the mechanics of chalices powers, I know it makes sense within the plot, but I would've preferred all of the blade riders to be able to achieve jack and king form, but alas. Anyway I personally would wanna combine Climax and Liner form, there are certain ideas I like from both but Climax form (to me) looks like more of a final upgrade than Liner form.
Cyclone Joker Xtreme needs some help, maybe have him sort of resemble Fuuto-Kun (which I personally think would have been a really fun "coincidence", have the final form debut and it just happens to share some common elements with the already existing symbol of the city. Not to mention if they had to use the "X" motif, i think a pinwheel would be a much better idea as it still ties to the reoccurring wind theme and is essentially a stylized X shape) Also get that sword and shield the hell away from him, he's a detective he doesn't need weapons like that. Maybe a gun or something, i think it would've been cool to just give him wind powers (sorta airbender-ish but have his movements not be so tai chi-esque)
OOO needed a more abstract final form in my opinion. With his gimmick the way it was I think A. All of the medals should have been able to work in any combo and that B. To just say one combo is the strongest in every way is boring. I think the strongest part of the OOO concept was the mixing and matching, eventually he never really had to cuz he had tajador and putotyra. I think it would've been fun for Eiji (NOTE: I realize this doesn't fit with the plot but just go with me for a sec) to somehow purify the medals and absorb one of each into himself and then be able to do instant full combo changes. Need to get at a yummy thats high up? fast as snapping his fingers he's tajador. Need a strength boost? Sagozo. Got an enemy hiding underwater? *snap* Shauta. This "final form" could look really plain (similar to Den-O's PlatForm), and call it like King Combo or something. That could actually be pretty funny, hype the hell out of this "ultimate power" and when he first gets it he's all bummed cuz its totally underwhelming, until he actually uses it and then he realizes its like mindblowingly powerful, i like that idea