Think about your favorite existing/official TF2 maps: what is your favorite theme? (Sawmill/Desert/Industrial/Alpine/Snow/Swamp themes are the big ones). Typically those are the most fun maps to make.
If you have the resources, you could even make your own theme, which is really cool to see it come together. I'm creating a semi-original theme for my upcoming map to replace my CTF summit in the contest (summit kinda sucks, trust me
), and it's been fun collecting the assets to make a somewhat new theme.
The biggest thing in whatever theme you choose is to make your map memorable and unique. Badlands is memorable for the spire on points 2 and 4. Sawmill is memorable for the waterfall and the rain. Egypt is memorable for...well, being unique. Viaduct has the bridge over mid. Offblast's mountain/spire thing just looks cool which makes it memorable, even though it's one of over 9000 desert maps out there.
tl;dr think about what you like in a map and most importantly make your map's geometry/appearance unique, even within a common theme.