I think riderless could indeed have some development potential and the direction you want to go can be very wide: riderless could be miniature or remote controlled models or vehicles that have nor rider...maybe a site on RC vehicles?
The ceremony of the Riderless Horse is an old military tradition. In some military funerals still today, a riderless horse is led in the funeral procession to represent the bond between the horse and its owner. It could be a site dedicated to cowboy history or something along those lines.
Riderless could also be a very cool name for a superhero too..a wild type who is his own man (or woman) and takes orders from no-one.
There are many outside-the-box possibilities not to mention it could be the name of a fashion designer's newest attempt at panties that don't bunch up!
It's a good one...if you don't develop it then hold it for a while. Too good to sell for under $xxxx.
Good Luck!