I'm probably 50% miniatures player/50% RPG player, but my mini purchases are more along the lines of 80% for wargames, 15% for RPG PCs and 5% for painting/collecting. The majority of my RPG purchases are fantasy figures, while wargame purchases are probably 75% sci-fi.
For RPG purposes, I've found card board standup "figures" are more practical for use as NPCs/Monsters/etc. Only PCs and major NPCs warrant a figure. I have way too many unpainted figures as it is to try and keep up a decent NPC stock of minis as well.
For wargaming, I now personally try to stick with armies that are usable across a couple of different game systems. I used to play a lot of 40k/WFB, but when I moved away from my old game group, I found that I ws going to have to update my rulesets to continue playing, or play another game. Of the armies I collected, only my Imperial Guard Army is generic enough to play with other systems. And even those aren't that generic. I'd really like to build a more generic near-futuristic human army that would be usable with a lot of different systems in the 25-28mm scale.
So what I would really want are basic trooper type figures in a lot of poses that are usable from anything from riot police, to security guards to infantry soldiers, and then some officer type figures. A really good range of those would go a long way to filling out an army. Take a look at todays combat armies - other than colors and insignia and minor details like weapon size/shape, it's pretty difficult to tell the difference between a british soldier in combat gear vs an American vs a Chinese soldier. When a mini is less than 2" tall, and there are a hundred or so on the tabletop, color can be far more of an important detail, as will be the vehicles.
Once you start going far-future, two things stand out that make things difficult - One, the introduction of technology like Power Armor and Mechs. I don't know a single science fiction fan who doesn't like the concept of Power Armor. But every science fiction universe has different concepts of what it should look like - I personally like Traveller's take on Power Armor, while some friends of mine are partial to the Star Wars Stormtrooper armor, while others are partial to 40K's Space Marine's take on it. So, in a sense, Power Armor is tied to the setting, and a lot of the more popular settings are going to be licensed property. The same can be said for Mechs. I personally can't stand the thought of huge walking robots, ala CAV or Battletech. I don't think we'll ever go there combat-tech wise - a box like tank with treads or anti-grav is going to be a far less complex and more efficient piece of machinary. I have friends who love Mech's though.
Two is Aliens. Unless you use the generic futuristic versions of the fantasy races like 40k and other games use, you either have to create your own aliens or license the likeness from someone elses sci-fi setting. There are very few interpretations of aliens that could be considered public domain, like elves are. Then, when it comes to the games themselves, you have to provide stats for any aliens you create, and they have to be different enough from the other races to really shine. This is why most settings/games only focus on a half dozen or so races at most.
So while I say I would love to see lots of sci-fi miniatures in power armor and aliens, it's pretty subjective - what I like isn't necessarily what you'll like, etc. Makes it pretty hard for a company like Reaper to even break even on sci-fi miniatures, especially when you throw in the fact that, sadly, fantasy dominates the RPG market, and generally has a wider appeal than hard science ficition. But I didn't ramble and bring up those points for nothing - I really want more miniatures in Power Armor and more Aliens. If I were in Reapers position, I would determine what to add to my line by sales, using something like this:
Have some concept art work done for a half dozen or so Aliens/Power Armor, probably by different people. Post them in the store on a pre-order basis for a month or so. Require people to actually commit to ordering and paying for at least one figure as their vote, with the knowledge that the only the top seller (maybe top two) are going to be made, and then only if their pre-orders will cover the cost of paying the sculptor and the mold. Once the month or two is up, send the top vote getter to a sculptor and go into production of it. Those people who "purchased" concepts that didn't get made can either not have their cards charged, or be given store credit when "voting" ends.
This could be particularly effective for Aliens - there are a lot of budding game designers out there who would love to have unique aliens not tied to any particular setting to use in their games. Plus, in any logical sci-fi setting with Extraterrestrial intelligent life, there should be hundreds of different types of aliens, not just the six to ten most systems focus on.