My current project is my first scifi vehicle. My aim is to make this an easy-to-build model, both for my own sanity and to convince skeptics to give paper model vehicles a try. I also happen to like the shape. Very Star Wars.
The scale is 28mm/30mm for tabletop wargames and RPGs. 1"=5 feet. Overall size of this model is about 7" x 7" and it stands 2.25" tall. If you were nuts, you could shrink this to 15mm scale, but I wouldn't recommend using the standard gun pods because the barrels will be too small. The heavy cannon should work okay at that scale, though.
This was my initial concept:
And then after a little more work, showing optional components and exploring some color schemes and roles:
With final additions of a power station, diagnostic computer and crew ladder.
I'm texturing this thing now, which is where I hope to add a lot of depth through panelization and weathering. The final kit will include PDFs with multiple graphics layers, so there'll be a couple of different styles for most components, plus lots of optional overlays (paint patterns, damage, unit markings) to tailor each model.