Okay. While I don't particularly like trains, those could work for the mecha. But Dinosaurs have gone their course.
I mean, what would you even call another Dinosaur season? Kyoryu has been used in both team and title adjective (title adjective being the word before "Sentai" in the title) so, what would say "hey kids! It's the fourth Dinosaur team! Awesome!"?
Maybe Toei could work together with Saban to come up with ideas, considering Saban has to adapt the season eventually, perhaps they could lay down some seeds for Power Rangers.

But I doubt that would ever happen.

Let's go for a gun fetishist Sentai where they are all based on different classes of artillery. Okay I'm kidding, although we all know it would be badass.

How about another video game theme? But base each ranger and their colors off of the different genres of gaming.

That sounds awesome.

Action Adventure game Ranger - Red
Racer Ranger game - Blue
Fighter Ranger game - Orange (We need some new colors here guise :D)
Shooter (FPS/Any Shooters) game - Green
RTS (Real Time Strategy) game - Pink

I don't have any ideas for an extra besides singing/music games...or RPG stuff.

Sixth ranger can be horror/hack and slash. And would obviously be black.