I really don't think that having a single Rider again will hurt the show. The little kids are not "expecting" a second or third Rider to show up. As long as the main-Rider can pull off a new badass form every week, then they are happy.
In my personal opinion, I think a lone Rider with power-ups is much more exciting than having secondary Riders show up.
With shows like Faiz, Kiva, Den-O etc. its like "Wow cool! A new Rider..." and then thats basically it... that Rider might get a power-up later on, or a third Rider will show up, but thats it. The excitement is gone.
When I watched Kuuga, I was excited all the way to the final, because I wanted to see what happened to Godai as he got stronger. There was always a new form or new power coming, and it never failed to dissapoint.