If you added ultimate regen per hit you'd be creating a very OP race, and given you already have to pay for the privilege to play as an Imperial it'll would be very controversial to give players a paid for advantage over others.

The problem with Imperials is that you can't really change them without angering people on either side because they are already a very powerful and flexible race; nerfing them will **** off those who have paid for the race, buffing them will **** off those who didn't paid for the race. Imperials already come with two top notch passives, Red Diamond is more of an optional passive, especially when compared to Nord passives (the closest thing to an Imperial otherwise). You can't do much with it without making a problem.

The best you could probably do is replicate what they did with Adrenaline Rush and have a consistent timer for when a hit restores health instead of the inconsistency with the 10% chance per hit.

If any Imperial passive needs changing though it's the Diplomat passive. 1% extra gold is the most useless of all those unnecessary extra passives they gave each race. Even if some of the others only work for a niche group of people, at least someone can find ways to take advantage of them. This passive literally does nothing and serves no one.