- Sports: I think there is a toku series from some country that did this, but it's time for Super Sentai to do the same. The sports that can be represented include baseball, football, soccer, basketball, and gymnastic.
It's a cool idea with a lot of potential, but there's one big thing it has to get around: Japan's most popular sport would probably have to be included. At that point, you have a hero who basically just walks around hitting people with a baseball bat. Am I the only one slightly uncomfortable with that idea? I'd bet Toei's lawyers would be too, as it falls into "an action easily repeated by children" territory.
On a related note, I think that the reason a proper "sentai in space" series hasn't been done yet is that having the action completely off Earth means you'd have to build new sets every week, which is both expensive and just not possible in the time period (we've finished episode 5, we'll be back in two days to start filming episode 6). So you have to either have most of the action on Earth (which is what they did with Dekaranger and Megaranger, the two sentai that got closest to a space theme) or a planet that just so happens to be more or less identical (and what's the point if the planet is the same?)
- International: I know we already had the Battle Fever J, but I think it's time to upgrade this motif after 30 years; the original even had the former Soviet Union as Battle Carrack! This time they can have Japan, U.S. (Power Rangers), France (a nod for France Five), China (Armor Hero), and maybe Germany or Russia.
There's a variation on this I've been thinking about for a while - ancient civilisations. I know Goggle-V technically did it, but all anyone seems to remember about that show was the rythmic gymnastics. Even the Gokaiger ending mentions that in favour of the show's other themes of ancient civilisations and gemstones. But you just assign each ranger a continent and have their helmet designs and weapons inspired by an extinct civilisation from that continent. Romans, Ancient Egypt, Aztec, and so on. You could save the Japanese samurai for the sixth ranger so you could get another from Asia in the main five.
I'll throw a couple more of my own ideas out there:
1. Sea creatures theme. Red ray, blue marlin, yellow seahorse, black orca, white shark. The mechs are all themed aircraft rather than actual mechanical animals. So white shark is like a white stealth helicopter, red ray is kind of like the Batwing from the first Tim Burton Batman film, and so on. As it's been difficult in the past to have proper hands on the mecha when a sea animal forms an arm, I'd have a Furaimaru-like system as standard where other, much smaller mecha (starfish, crab, lionfish etc) are used to form the heads (or at least helmets) and hands of the combined mecha. The rangers themselves have taiahas as weapons.
2. Rangers themed after kinds of energy. Red thermal ranger. Blue electro ranger. Yellow kinetic ranger. Green gravity ranger. White light ranger. They have individual powers based on their energy theme, so red can make things very hot (or incredibly cold), green can increase gravity to pin enemy mooks in place (or remove gravity so they float up in the air) and so on. Later on they add a black sound ranger to the ranks. The enemy boss is nuclear energy themed.