Well here's the problem: we don't know what you want, how the other parts of the map look/will look like, etc.
Walk around other 5CP maps, and see how they've done it. I'd recommend taking a look at Granary, Process and Glassworks (the last is not an official map but it's a good map). If you want to know how not to do it, look at 5Gorge and Fastlane and see what they've got in common.
Also: welcome to the TF2maps.net community! Say hi here so we get to know you a little bit more.
If you don't mind me asking, what is the issue with cp_fastlane? I personally find the theme forgettable and bland but Im not sure that's what you're talking about.
Tips before looking around 5cp mids:
DON'T look at 5gorge's mid
DO look at granary's mid
DO check out badland's mid
Mid is more or less ok.
There's absolutely zero easily accessible high ground at mid. It just generally leads itself to very passive play. It's boring, to say the least.
The high ground that is there is only accessible by explosive jumping, and the huge height advantage plus the health kit up there makes it incredibly easy to hold onto. It also makes it harder to push back in.