Not completely linear of course, but... more flat than now.
I think it's pointless that you make the first 15 to 20 levels in a couple of minutes. And, what's worse, after hitting level 37 it's basically over, unless you want to grind another 500+ million. I'm not saying you should be able to reach higher levels, just the progression shouldn't stop so abruptly. Levels should be gained more evenly throughout the xp gain imo.