So, well, this isn't entirelly developed yet which means I'm open to suggestions, the basic idea would work like this: 

Shocker, the organization Kamen Rider 1 and 2 fought years ago, and had been rising back here and there in movies, showed up once more, along with all villanious organizations and groups of KR. The riders once more gathered and fought them fiercily, what unfolded a war which lasted for 3 months. The Great Leader of Shocker, along with his generals in the end were all defeated and sealed by the very powers of the Kamen Riders, never to show up again...or so they thought. 100 years later, the seal which held them was broken and they were released, however so were the Rider's powers, the problem is, the former warriors are now either gone or too old to fight and new bearers of the belts must show up. That's where our characters would jump in, either as the new bearers of an already existing identity. (For example, the new Kamen Rider Decade....ONORE DECADE! D<) or entirelly new riders using the powers of the old. (Like a rider who uses core medals like OOO, however doesn't use Taka, Tora and Batta as his standart medal combo.) 

I've been thinking about this for a while and I'd like to see if anyone would be interested.