One thing I like to do is vary the density of pieces on a level. It's very tempting to fill up a piece with little details. How about taking a look at the level and deciding how much density would balance or unbalance it in a visually interesting manner? Sometimes minimalism can give everyone some breathing room. Breeding vroom!

There's nothing at all taboo about doing two or three conjoined pieces simultaneously. Clever reservation tactics can insure you aren't artistically ambushed before uploading your gridchunk. Take more space to show what you want to show.

More space = more room to show your style.

For what it's worth, I don't find it interesting to fill up an entire level (or two) on one's own. It's not technically tsk-tsk'd or even verbally verboten to take up a level entirely. Do it if you want to, but don't be surprised if you're called a ball-hog.

Other suggestions for varying level aesthetics: agree on a photoshop tool to use exclusively for a specific level OR use only square-marquee cut-n-pastes of previous levels OR decide on two-color scheme OR ...?