Achievemtn rewards to increas energy slots.. Like 5 e per level or 10 e per level.

Supporter or subscriber gets 200 + 5/ 10 Energy per level

Normsl gets 100 + Level 

I think 5, 10 and probably even 2 energy in a purchasable capacity would probably be somewhat frowned on, but the idea of +1 energy per level has some interesting implications (and probably would also be frowned on) but it is fun to think about. I would love to give more reasons for players to level up.

The original suggestion is way too OP but I do like the suggestion that vaelinn made about enticing a player to level up. The gain would have to be small though otherwise you're also entering that realm. Something like 2e every 5 levels? Still gives you an extra 40e at level 100.