Go all out, make it ultimate power fantasy. Everything is overpowered. Throw balance in garbage bin.
On more serious note, ditch classes. Point buy, fixed point budget depending on type of power level you want. So it can easily scale from grim and deadly to superhero. More flexibility for horizontal growth, not just vertical. Abilities have points cost, stronger it is, pricier it is. Every level, you get fixed number of points to buy abilities. It's up to players to decide if they want to be one trick pony, but really powerfull in that one thing, or they want to have more abilities that are weaker.