I've been thinking of a situation that might be quite similar to Eternum. It's LARPing.
When you LARP you can be the Friggin' Archmage of This Prick or the Invincible Warrior of my Ass, but if you are a lardass or a 50 pounds stick you will still be a lardass or a 50 pounds stick.
Now, i've been larping for like 15 years when i was younger, and you can bet, i've never ever been a fit dude. I'm big and strong, but i'm also very fat, which mean i've never been quick, fast or agile. Yet, i've always been a rather competent fighter who could kick asses more often then having mine kicked. I had to play smarter and with more strategy, since i wasn't fast, but it worked smooth enough.
So, why not in Eternum? I mean, you don't need to play a ninja all the time. You could do something more in line of you natural abilities. Maybe you can play an archer or a snyper, rather then a mage or a techie. Or you can play that damn ninja but being smarter then fit.
At this point levelling would be more of a matter of access to special powers, abilities and equip, rather then grinding your way through NPCs.