Crystal will add color and body to your beer, as well as some unfermentables. If you aren't looking for sweetness, I probably wouldn't go there. Certainly you could balance it out with more hops, but then you are moving to greater complexity, which you say you don't want.
I think a better option would be to steep some Munich or Vienna malt for about 1/2 hour at 155. This will give you more fermentable sugars, but won't jack up the alcohol too much. It will darken your beer, but only slightly. I think that would likely hit the additional flavor you are looking for.
Just adding more extract will just make your beer stronger. Not necessarily a bad thing, but not what you say you want.