Henshin Justice UnlimitedTokusatsu - For Great Justice! Fire up your giant robot, get your changer ready, and strike a pose! Here's were we jam on Sentai, Kamen Rider and other Super Entertainment from the Land of the Rising Sun.
Ryuki - All rider survive form
Faiz - Kaixa Blaster
Blade - All riders King and Wild (only Chalice)
Hibiki - 3 Armed
Kabuto - All hyper (kabuto & gatack both using hyper, thebee, drake & sasword hyper borrowed)
Den-O - Zeronos zero form makeover
Kiva - kiva-la empress form
It doesn't hurt to make him even more overpowered than he already is.He can basically assemble an army on his own and he's probably the only secondary Rider stronger than the main Rider.
I think you're kind of overrating Diend. He can only summon 3 Riders at once outside of Complete Form, they've been shown to disappear after taking limited damage and there are some clear limitations too like no final forms and his Kabuto Riders never using Clock Up (and Diend himself saying that he didn't have it while in the Kabuto World).
That is to say form changing and ultimate forms should not be a requirement.
But if it's gonna be there then I do think the forms and the ultimate should not look like a gaudy Bandai hotmess or take away the things that work about the heroes main form design. To me Kuuga is the best example of this (not counting "Rising Ultimate" which looks like somebody let Rob Liefeld in through the back door at PLEX.)