I do have another idea that involves fusing the worlds... but I would need cooperation from everyone who's still active... and this next part is inspired by RK's idea.
We'll fuse 2 worlds and have have only the characters from those worlds interact in a side story... fighting each other AND/OR fighting the ultimate threat of that world. The episodes should take no more than one week if we can move this steadily, or 2 at most, depending of activity.
How this begins and ends will be up to you guys but once the rider war is over, the worlds and characters will reset. So within this story... if you're willing, is if you aren't the MAIN rider of that world, you can die in battle... because once the world is finished, it'll reset and we can move on casually. It'll seem like a weird move, but it'll have the "what you don't know, won't hurt you" motif.
RK... that Children's Card Game (insert name of reference) =P sounds a lot like when Yu Gi Oh went to Dungeon Dice Monsters or Capsule Monsters for a short period...