[smt1923]

Well, considering all of the hints they've been dropping Sentai: Western themed mecha formation, a sixth Ranger whose monster form resembles a cowboy, a sixth Ranger who is a ninja cowboy with a cheeseburger cellphone morpher (god, there's a combination of words I never though I'd say), I'd put 50 bucks on the next motif being Cowboys, possibly something in the vain of Gokaiger.

As for other ideas:
Insects (Kamen Rider has been riding that for over 40 years, so why not Sentai?)
An all female team
Another car themed Sentai
Sea creatures
Monsters (yokai or things like vampires, ghosts, etc.)
Zodiac (hey, if Kyoryuger can get away with 11, amiright?)