For years, I have wanted to do an aged warrior who is picking up the accouterments of war one last time, a glory ride, if you will, for a just cause.
I've written it up in D&D as a Ftr, Rgr, Brb and Paladin, with a variety of reasons as to why this "veteran" is mechanically low level. In some write-ups, the warrior is merely quite old, in others, dissolute living has left him or her a shell of the former stalwart. In each case, skills have atrophied, talents diminished, and powers have been forgotten. As the party adventures, the rust is shaken off and those abilities "resurface."
Some incarnations go a bit Quixotic, with the warrior having a sidekick who- hopefully NATURALLY in the course of the adventuring- begins to emulate his master, and gets to take over his mantle & quest when the mentor ultimately fails.
But ya gotsta have the right GM & campaign for that...