Sorry, but you pose a very silly question. You're obviously neglecting one of our most effective tools: reference. Go dig up as much futuristic scenery as you can possibly stand. If you're a creative person, you'll get plenty of ideas on what to do. Judging by your tiles, I assume you'll have no problem once you get out of the box you're stuck in. Take off those tunnel-vision blinders and go embark on a visual journey, surf randomly, find lots of useful stuff. It's certainly out there.
On your tileset, I think it's great. Fairly interesting in and of itself, well-enough pixeled. Though, like you already acknowledge, it's not very exciting, all very boxy, lacking exotic quality, buncha shipping cargo container block. But that's not a bad thing really, you just don't want to overuse the set, that's all. It's monotony will bring focus to your more outlandish sets.
I find your sky bg equally insipid. It doesn't suggest "futuristic". Only thing I can think of to save it would be to animate lightning illuminating huge portions of it at random. Can't go wrong with nifty light effects.
