One problem with level-based systems (and, let's face it, any system where characters progress is in some way, shape, or form, level-based) is the uselessness of low(er) level chars.
How to remedy this?
I recently came up with this idea: why not put the quality of drops and the level of the PCs in an inverse proportional relationship.
Okay, let me illustrate what I'm thinking of:
You have pretty low-level mobs. Perfect for low level players. Now, they drop some resource that players of all levels need (plenty of) for some (or many) crafting processes.
Usually, this means that higher level characters would wipe that population out regularly.
So, let's give lower level players higher rewards. No, not XP. Let the resource the mobs drop be of higher quality if the player killing it is a lower level.
That's game mechanics, simple and easy, higher level player, lower quality.
But how to rationalize it in-game? How to... RP it? Well, let's say it's some secretion the mobs (some sort of animal, for example) produce. And they only produce it when they're aggressive. And they only get aggressive when they see a chance at winning.
If on the other hand they see their packmates getting slaughtered by a high level character, the run in fear, and do not produce that secretion (or in lower quality).
Think of it as the difference between "fight" and "flight".
So, the key to getting high quality items of that resource would be to kill these animals with a lower level character.