maybe a prestige or reputation based mechanic which taps into trophies which the player collect? each one would have a story which uses key words that tie back into the narrative elements unique to the player and/or group.
and since you are basing on sentai, you could have special power items with limited number of charges, or time between recharge (or some other limiting mechanic) that they can add to their getup. a crystal, complementary piece of jewelry, something that merges with them for a slight thematic change, again using added or expanded narrative elements to tie it back into other parts of the game.
perhaps instead of items, you could allow them to use a fate point system tied into soliloquies, where the player(s) recount a story of a character victory, or a lesson learned from a defeat, and the associated fate points can then be spent for heroic efforts above and beyond their normal ability or some such. perhaps a given soliloquy can only be used once every so many game sessions to keep the stories fresh - this would also have the potential to engage the players in collecting story and narrative events for their character.