I have a few ideas, mostly personal opinions of mine that fill a particular niche:

1). Kaiju RPs - My attempt at kaiju RPs hadn't been as successful as they should due to my currently sheer incompetence in directing them. I have an idea, a vision, but often lack the sudden will or time to do so. However, I feel like I have a few good ideas with them - such as controlling Monsters rather than simply just people. 

But while I'm a fan of the whole crazy tokustastu culture, I wouldn't mind something akin to a "documentary-styled" RP where it focuses primarily on the monsters and how they interact with the world around them and with other players. I'm not the only person to have this idea, but even just creatures in general, have a RP that focuses with monsters or something... 

2) Which leads me to maybe have a solely dragon-based RP. You take control of a dragon and interact with others of your kind and maybe engage on personal missions either against other dragons and/or against humans. Basically it's a reversal role of you controlling the creature against humanity.

3). Space faction RPs. Or in my case, multiversal space RPs that allow you to control a ship or faction from pop culture, with a cool original story that can entice and rally other players. One RP I partook in had you control a faction of your choice and you could pick where to land on a map of a custom world, where you could expand your forces and interact with other players. This is something I would love to see more of!

4). And I wouldn't mine more Monster Hunter or Monster-Hunter-based RPs to, also with the akin of being able to control monsters in "ecology-styled (cutscene)" situations.

So to wrap it up:

1. More Kaiju/Monster RPS
2. Documentary Styled RPs based around nature and so forth.
3. Space Faction RPs
4. Monster Hunter RPs