
After deciding to go for a simplistic level-up system and simple items as well, I'm suspecting this might be a good way to get the game a bit more interesting. Certain areas of each level might be locked so coming back later will reward you. Perhaps you'll get an item or even a sub quest. Depending on how complicated the level generation algorithm is going to be, these sub quests could either be locked to that level or they could contain different parts in different levels. It's of course important to make sure the level generator doesn't cause a deadlock situation where you need A to get to B, but you need B to get to A.
In Zelda you either get items or upgrades to get to new areas. I'm expecting upgrades might be the best way to handle this in Soulreaper because of the simple item system. You could for instance get new skills by being trained by a mentor, helping a demon or perhaps sucking the soul from a seraph.