Oh I leveled her to 10 like a bunch. The ones I unlocked are Captain America and Iron Man in the steam sale, and Deadpool. I have another free unlock to use.
For the cake slices I got a fifth stash.
I didn't find her phasing move in combat skill useful.
But I do find you run out of hotkeys with the 8, not convenient to use the other bars.
I hate games that want you to use more than 8 skills, like WoW where you just put everything on the screen and aren't really restricted from using whatever you want.
This annoys me with some of the Marvel Heroes, but it's very few heroes that you need significantly more than a single bar--Dr Strange requires a little more than 2 bars to be effective.
For this, there are some scripts where you can hotkey second skill bar slots to new key bindings.
I've' mapped my keys like this:
Skills 1-6: asdfer (e & r are basically my money shot/signature rotation: depends on the hero whether or not the sig is found there).
2nd skill bar I place the always-on auras for the heroes, things I don't have to activate, but slot 1 and 6 are for my 2nd hotkey skills, I fix those to w and t.
This gives me 2 extra skills beyond a single skill bar that I can quickly activate without needing to flip bars. I find that to be more than enough for 95% of my heroes. There are other scripts that people use for toggling skill bars and hotkeying even more skills, but I like to keep things simple.