Yeah I don't see any need for levels to go above 20...none of my skirmish games have I gotten above like...15. Anything more and you're really dragging the game out. It's like complaining that the scoreboard only shows double digits in a football game that's currently 104 to 32.
Ive already managed to get to lvl 20 several times, including in the pantheon. During long matches such as dominate its nice to be able to level over 20. Simply so it wont come to an halt. that the levels stop at one point during the match, and if you've worked hard to level, other people will still catch up to you just because you cant go any further.
It doesnt neccesairly have to be new skills, can be more strength etc. Or you could simply spread out the skills a bit more so you can choose simple attribute skills in between being able to upgrade your current skills.