It should be noted that contracts ARE longer than they've been before since the current cast members return a year after their series ENDED for a VS movie, and there might be even a Returns or something else included in it too.
For Riders, there's the Movie War, a couple months later.
It's not a full extra year of a regular 50-episode series, but it's still longer than it used to be.
How did it work with Black RX? Did Kurata sign for 2 series, or did he sign up again for a new one afterwards?
As for me, what I'd like to see it's a sort-of-passing-of-the-torch. No, not the stuff they do when they're showing the sponsors at the end of a series.
Have it be similar to what Kamen Rider V3 did in the beginning of its series, and what Power Rangers Turbo did in the middle (was it the middle or earlier?): Have the previous protagonists be around to help out the new one a little bit before they set off, preferably not giving up their powers but fighting evil somewhere else.
Or my dream way: Someone that was a regular or semi-regular in the previous series, but not the main character, becomes the next protagonist. For Rider it could be a secondary one. For Sentai I'm thinking back to the Turbo example. It's some people that were around, preferably helpful to the Sentai team, that end up getting their own powers. Think also of Yayoi in Kyoryuger but instead of becoming KyroryuViolet she becomes a main character, as another Ranger, in the series after that; or alternately she can also become KyoryuViolet but still gains another power as part of the main cast of the next series.
As for the shared universe aspect, Sentai, along with the rest of the Ishinomori-verse at the time had it for JAKQ and Goranger. They only teamed up in the movie but it was still a pretty solid presentation of a shared world. Along with Robo Detective K, I think Inazuman and the first 5 Rider series (assuming it's all of them based on V3 and Amazon's mentions alone).
The first 5 Rider series were all about this. It can be attributed mostly to Hirayama Toru, who was producer for all of them. Once Skyrider stopped being a reboot and brought back Stronger, it was the pinnacle of that era, which culminated with ZX. I like to call it the Hirayama era... I think I borrowed that name from Igadevil.
The main thing to point out is that whenever you watched a new show, it felt more like a new season and less like a spin-off (at least to me). And it left a heavy mark on me, so much that I think of a new Rider show as a new season, and never once consider the possibility it takes place in its own universe, separate from the other series, unless it's explicitly stated.
That is the exact same feeling the Power Ranger series from MMPR to In Space gave me. I've only considered RPM to be in a separate universe so far.
It'd at least be pretty cool to have the previous protagonists appear in the new series to help the current Rider/Sentai (as it used to be in the Hirayama era, with the exception of Amazon and Super-1). But, we do have them in movies and specials which is at least an improvement over the isolated format they adopted for some time.
Not that I'm saying a stand-alone series is bad, I just go crazy for a shared universe. Hirayama Toru and Judd Lynn are to be blamed.
I remember somebody mentioning Gavan -> Sharivan did something like this very well. I haven't seen them, can anyone confirm that?