I've been thinking about Horizon Wars and wanted to make a post much like the OP, hence the bump. The Pendraken sci-fi range looks fun so I think I'd go for that and mix in soime moderns or what have you. I like the Pendraken aliens but without shooting I'm not sure how they would work in HR. Ideally I'd like to see a hi tech force with low numbers of elite units vs low tech force with more but less effective units. The 'pathetic aesthetic' appeals to me at least as much as elites who barely fail a dice roll. Probably linked psychologically to many defeats to Space Muhreens during my formative years.
As the choice of 10mm sci-fi is thin on the ground I'd be interested in what 6mm or 15mm stuff would successfully scale up/down.
EDIT: the Vanguard 6mm range looks so good I think I could go exclusively for that. Even their 3mm range looks promising but they need to go beyond the one race/force they currently offer. Apparently they are going to see how they sell, so fingers crossed.