Not everyone would have uber creatures, this would just unbalance PvP more.
You lack vision.
This idea is not about making overpowered cards, it's about making
balanced cards. If a creatures has an effective ability, it would cost a lot more to play.
A lot of people here seem to think that one 100/100 creature with abilities is an instant win. This is very dumb logic imo. You have to realize that these kinds of cards would be
very expensive to play, and would be owned by every single speed deck out there.
Momentum is the biggest problem because there is almost no way of stopping a creature from attacking at least once. This is why it should cost more than other abilities. Best thing would be to give every ability a cost based on its effectiveness and/or the cards Attack/HP (for example with Dive and Devour)
The more I think about this idea, the more I like it. It's would be a nightmare to program, but would probably turn out to be something amazing. It would surely give a new meaning to "building your own deck".