I would love another military/organization Sentai again. It was rumored that the ToQgers were train themed and a rescue team, similar to GoGo V, and their civilain-wear were like conductor uniforms. I gotta pretty excited about that, then I was a bit saddened that the ToQgers were immature amnesiac young adults..but whatevs it's an entertaining show.
Aside form military/organization, I would LOVE for Toei to do birds/avian themed again. I'm actually surprised Toei hasn't dont birds again considering how popular Jetman was and a real classic, considering we've had 3 Dino themes, 3 car themes lol.
And also insects, it would work so well! I was told that Sentai has never done insects because of Kamen Rider, but aside from the usual insect inspired helmet designs(eyes, mouth or antennae), Kamen Rider hasn't explicitly done the insect theme for awhile and Kamen Rider has even used Sentai themes; Wizard with magic/wizardry from Magiranger, and cars(Turboranger,Carranger, Go-Onger) with Drive now lol.