Hey Tragik, welcome to 40Konline and the Space Marine cult
Your models look awesome! I like the shoulder pads and the armor highlights. Good pictures too!
I played a little Warhammer Fantasy back when I was in high school but never strayed from the goblin green / flock bases. I'm quite a bit older now, and my hand is steadier, so I thought I'd try to make everything look as nice as possible haha.
That's very recognizable
when I see some of my old fantasy models I can't believe I did the bases like that!
Basing is actually very important for the look of your models. The level of your painting deserves some good basing. You're right about avoiding dark basing because you need some contrast with the grey. But if you wanna go for a city look that's a bit difficult. You could make it a very bright, clean, marble city with very light colored streets.
You could also consider doing a bit different themes of basing for each squad; but not too different to it's still coherent.
Good luck! Hope to see your future posts on your painting, fluff and army lists!