Luckily there is are rulesets which encourages solo play and role-playing : Two Hour Wargames "5150" and AmbushAlley's "Tomorrow War" (to be published). I expect house rules to follow.
The theme is simple: Colony 13 is located at the exact point in space-time where every science fiction story ever conceived by man or dolphin intersects. Imperial Stormtroopers versus Giger's Aliens? You got it. Plus BSG Centurions, Daleks, District 9 "Crusties". They're all there. Trapped in a pocket universe that they can't escape. :-)
I chose 15mm so that I can play easily on my cramped kitchen table --- most battles will be on a 3'x3' board. The idea is "game in a box" -- I want to be able to grab the box, set up and play through a game and put it away so that the cats don't eat it -- all in an evening.
The cool thing about choosing 15mm is that I have ZERO supplies for it. So the colony will grow as I acquire terrain, buildings, etc. And I hope to document as much as possible in a webcomic kind of way. Oh! And I've set a $25 monthly budget limit on acquisitions although I will have to be slightly flexible in this to save on shipping costs. So I've decided to combine July and August to kick things off. (So that I can get two sides to fight!)
One other goal is to spend as much time on the terrain as I do on the miniatures. It's a doll house with Aliens. :-)
It's a grand experiment. And the first miniature will be on the way in a week or two (the company I ordered from is waiting on a restock, which suits my timeline just fine).
PS - Oops! I just realized I do have some 15mm stuff -- for AK47 Republic, battles in Africa. I might be able to use those once terraforming of the colony has started. :-)