I thought 'Super Vegeta' was USSJ, you see him ascend from SSJ to USSJ towards the end of episode 140 - Super Vegeta. In this form the muscles only swell a little bit and nobody really mentions the speed decrease. Trunks also ascends to USSJ.
When USSJ Vegeta is fighting Perfect Cell (before the cell games are announced) he is getting beaten pretty badly, Trunks tells Krillin he has surpassed his father but doesn't want to show it for fear of damaging his fathers pride. He waits until Vegeta is on the brink of death (and unconscious) before flying up and beginning the ascension.
This is the last third of episode 147 - Trunks Ascends. His muscles continue to expand until his arms are thicker than his legs were at USSJ. It looks pretty ridiculous because his head looks so small in comparison, but the first punch he hits Cell with causes him to roar in pain and his eyes widen. After the first few punches Cell begins to dodge and Trunks realises his every attack is missing. Cell beats him and announces the Cell Games.
While this is going on Goku and Gohan are training in the Hyperbolic Time Chamber, Goku is SSJ and calls Gohan over, he ascends to USSJ, his hair is a bit more erect and his muscles grow. ("Takes it to the next level") and Gohan says how much stronger Goku has become.
Goku is very serious and says "I could go one stage further if I wanted to." He roars and his muscles grow
again, very closely resembling Trunks when he ascended in the episode of the same name. Gohan says "I know you can do it! I know you can beat Cell!" but Goku says "This form has a weakness, these bulky muscles make it too difficult to move, whats the point in having this power if I can't even catch my enemy?" and decends to regular/normal. The form is never
called USSJ2 but it is stated to be a different form by both Goku and Trunks.
USSJ and USSJ2 are both flawed versions of SSJ2, when Gohan ascends to SSJ2, Trunks says to Vegeta, "He did it, he ascended without losing his speed!" and from then on USSJ2 is never used again.
I think it would be useful and cool to have both USSJ and USSJ2 even if it does make the player lose speed because if you can cope with the speed loss then the power is a massive advantage, then when you have enough power to get to SSJ2 you could lose the ability to go USSJ (why would you want to anyway, it's just a slower SSJ2).
As much as I love the ideas going on here, I strongly disagree with attacks draining health.
Kaioken isn't really an 'attack', it would be down to the player whether to sacrifice their own HP for a temporary increased boost of power + speed. Maybe if you used it just before starting a bout of adv melee you could really take advantage of it.
Tien can use kaioken as he trained with king kai, who taught goku
It would be good for him to have something to transform into other than the 4-armed manbeast. My only niggle is that he only uses the 4-arm tech once ever (in DB) and after that he never uses it again even when fighting for his life (Cell Jr's), is there a reason behind that?