Decade's Kuuga and the original Kuuga have basically nothing to do with each other.
His initial development was obviously meant to make him closer to the original Kuuga. He goes from "You fight for the smile of a girl? Your story is boring." to "If I protect everyone's smiles... I'll be much stronger!" and generally followed that afterwards.
He ends up pretty much like a more immature Godai, trying to befriend and help the Riders of the various worlds, although he still had some extra failings. That role was especially important in the Kiva, Ryuuki and Blade Worlds.
Their powers seem only broadly similar-- Onodera never demonstrates things like super-senses in the Air form or super-speed in the Blue form--
Decade-Kuuga showed the super senses of the Pegasus form in the Kabuto world, but, more importantly, Onodera himself used the increased defense of the Titan form to completely nullify the attack of a Mantis Lord in the Amazon World.
and it's totally unbelievable to me that DCD Ultimate Kuuga is as powerful as the original Ultimate Kuuga.
And I really can't understand this position. Its true that DCD Ultimate Kuuga's replaced the spontaneous combustion with spontaneous... floating...? But, aside from that, the battles of both Kuugas had a similar style, just a pure physical beat down.
It's not like the original Ultimate Kuuga showed some over the top special effects to show his power. His power was just implied by the story and listed in statics from outside the show. Kuuga's battle against Decade in Natsumi's dream is pretty much the only example of a "flashy" Ultimate Kuuga.
I can't help wondering if she's going to have something going plot-wise with Ankh? The opening seems to hint that she'll get more involved in his story, beyond "oh no this terrible monster has possessed my brother"
I'll be surprised if she becomes relevant. Back in Den-O, Hana had every reason to become relevant actively, rather than just as support, considering her status as a singularity point, but, even before her original actress left, that didn't happen.
Hina won't become a Rider in the tv show and I can't see her having a connection to the plot bigger than the one she currently has. Even if they reveal that her super strength is due to a medal in her body, that would just lead to a story about a Yummy possessing her, not her using it to help OOO.
She might end up having a more active role helping Eiji, like possibly arranging a fake marriage with him in Movie Wars Core to attract some enemy, but it's still just a supporting role... And Movie Wars Core isn't even written by Kobayashi, something that throws into question the idea of her becoming more relevant in the tv show. Unless she is actually going to marry Eiji for real, and I have no idea about how they'd handle that development in the tv series.