I thought Decade's world jumping concept was perfect for a Heisei Rider crossover, and the only way it could be done without insulting the viewers (although some still feel insulted by its execution), but I'm not sure about Sentai...
Even though they pretty much have to be separate worlds, there's already a big continuous line of annual crossovers, which can't be just ignored. Considering that, here's my idea...
->First 10 or so episodes introduce the new team which have their own enemies. The enemies receive reinforcements from a mysterious warrior in the 5th episode.
->10th episode - It turns out that this warrior is actually member of a super villain group and it reveals itself, completely overwhelming the new team. That super villain group would have various divisions, like machine, alien and beasts. All fully original villains, not reused suits.
-> 3 or so previous Sentai teams appear to rescue the current team. After the battle, they explain the situation, about how there's an invincible evil force looming over Earth, and it's unifying various evil powers under its banner, to counter that, the previous Sentai teams joined in an unified army and were fighting a secret war against it through the years.
-> New team joins them. From that point on, almost every episode will involve the new team being sent on missions alongside one of the old teams, with the villain of the week resembling a villain of a previous series. If possible, an actor of the previous team returns to interact and give character development to the new team.
-> The initial power of that team is a fully original force, but they eventually get power up and mechas that are focused on the Sentai homage aspect, after joining the Super Sentai army.
-> Finale would involve a worldwide battle, with shots of every Sentai team fighting all over the world in separate locations (to avoid showing everyone together). At the end, every Red would team up to take down the final villain.
Now you could just do the All Riders route and then have just guys in suits with voice actors and no human form for the past Sentai but then you would have complaints about no acting necessary.
If they went with an idea like that, most of the time, they could just choose one Sentai member from each season and focus on them, the others restricted to suits. A few seasons probably would end up as suit only though, considering how this is Toei... Anyway, same could be applied to my idea over there.