I know its fairly obvious, but it doesn't seem to have been suggested yet.
Some equipment could have a level limit (e.g. at least 20 levels in Taijutsu)
Other equipment could have a stat limit required (e.g. at least 12 strength to wield this big sword)
And some could have a combination of the two.
Of course, with you being able to untrain stats and leveldown/levelup levels this could be abused, but of course that just means that you have to choose which stats to have with which great eq, and add another dimension to the stat use.