first of make the game play cleaner or neater. this includes fixing glitches/bugs prevent freezing or making it run faster.
make more ability for characters so higher lvls have a goal.
add more classes like:
rogue:low health high damage moderate mana
paladin:damage fairly high high mana moderate health and healing abilitys
honestly there are so many class suggestions these are just two i liked.
more monsters and a final boss. the tree of monsters starts with an ancient tribe to undead to demons with elementals some where in there. there should be more of an obvious theme for each string of levels more monsters and monsters like mutant or earth elemental should appear more than once.
