I'd honestly quite like to see another ninja themed sentai, I think it'd be interesting to see a ninja theme sentai with some heavier themes like the samurai/growing up theme in gaim.
But if I was just going to try and pull something out of thin air...hmmmm...I think it'd be interesting to see a sentai that was themed around a marauding band of warriors. I've always wanted to see a sentai where the individual suit designs had a bit more individuality while still being instantly recognizable as part of the team. In a way, story-wise, I'd imagine it sort of like gokaiger, in that the characters are a group who came together by convenience and common goals, and in doing that slowly build a bond.
Visually, what i mean with the suits is that they'd each have parts of their design that were unique, one would maybe a have a longcoat, some rings, and a sword, while another might have some kind of hat, a cloak or cape, and a staff, another might have a vest, a scarf, and some kind of gauntlet-weapon. Etc. Etc. With each of them taking on roles that were sort of like characters from rpg's. The lone swordsmen who doesn't talk much and has a tortured past, the cocky hand to hand fighter who just came along to show off, but secretly has alot at stake, the mysterious mage, imparting through stories and guidance, the healer with a secret power they dont yet understand, all that kinda good stuff. I even think this could bring a cool idea to the collectible angle, they could have the items they use in the show based off rpg items. Things like hp and mp potions, a teleportation item, a few instruments, some weapon upgrades, maybe even armor upgrades, items that boost stats, keys, various loot.