Character Level and Energy Cap
How about each level also increase your energy cap by 1?
Right now, character level isn't very useful. I think it only increase the level of the monster you captured.
Boost energy cap really doesn't hurt the game balance. You still need to wait 5 minutes to regain 1 energy. The higher cap only allow players to stay out of the game longer to focus on their real life work. (The daily free energy can stay at 120)
Maybe it will benefit those who used gold coins to refill a little bit, but even at level 30, you only get 30 more. Besides, the refill is progressive (the refill at 10 gold coins, next is 20...). So I doubt anyone will refill too many times. Even 10 gold coins for a refill is too expensive already. That extra 20-30 energies probably won't be enough incentive for people to abuse it.