So would maxhp *= 1.2 or something do?
Most RPG's also make leveling so that it will become increasingly harder at higher levels, something like:
(currentlevel + 10)*(currentlevel + 9) for a nice effect, the number would be the XP needed to reach the next level, 90 to reach lvl 1, then 110 extra to reach lvl 2 (200 total xp), etc..
Some RPG's use only that, others only decrease the XP earned, others do both, it makes sense that a higher level character does not earn as much xp from a low leveled enemy as does a newbie after all.
How does your MP/HP system work? Do you want it to regenerate? If so then you should probably only bind the amount of HP/MP to the level, and make the regeneration speed upgradeable with special items etc. ofcourse it makes sense to have upgrade items for the amount of HP/MP as well.
Up to you though, your game
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