[Matrixbeast]

Aesthetically, it just comes down to the design choices. A bulky final form can work well even if the initial suits were slim if designed well, and vice versa. When it comes to accessories like capes, I feel they only work when the show had a precedent (Such as Mugen Ghost's trenchcoat like design working because how it's a natural feeling upgrade from the normal parka ghosts).
While Kiva didn't have any free flowing parts, I feel Emperor works because the cape like portions work well to emphasize the regal nature. So, basically, if the final form has an aesthetic accessory, there needs to be a point to it, not just tacked on.

As for other aspects, I feel the biggest thing a final form needs is a drawback. A reason to not use it. Kiva Emperor is infamous for being the most used final form in the franchise, and the real world reasons aside, in canon, there was no reason to not use it over his base form, as it was just a straight upgrade. The same can be applied to Hyper Muteki in Ex-Aid.
What helps keep things interesting is high stakes. And when the hero gets access to a powerful new ability that has no drawbacks, the normal form becomes obsolete.

The drawback doesn't always need to be life threatening, mind you. There's nothing wrong with having a drawback that could be overcome later on. Like Eiji managing to control PuToTyra by not letting pure instinct take the wheel. Just using the final form doesn't have to be a life or death situation, but just place some reason to not just use it forever.