If we're just listing what we'd like to see, the two main things I've always wanted to see rider based around are:
1. Cyberspace (Might ever happen)
A rider series that deals with the idea of the internet, software and hardware, hacking, viruses, all that good stuff. But with some kind of strong emotional theme like how gaim tackles the idea of growing up.
2. Jungian Psychoanalysis (Probably will never happen)
A rider series where the riders deal with situations involving the archetypes seen in jungian analysis, an obvious one being the hero(Containing the elements of a persons psyche which they consider virtuous), with other ones being the shadow (Containing the parts of a person that they cannot come to accept)The anima/animus (an Opposite gender being which represents qualities you may lack, but find attractive) or really many other archetypes, monsters and riders could be based on literally anything, but would be derived from those base characteristics. It could be done alot of ways, but I'd like it really surreal, with lots of dark stuff, but also lots of light-hearted stuff to balance it. Alot like gaim in ways, but rather than focsing on kids growing up, it would focus on people of various ages overcoming emotional struggles from past traumas. Sort of like wizard if the monsters had been more like the imagin, in that they come out of a person, or like the yummys, but rather than representing that persons greed, they'd represent their shadow, or some other archetype running amok.