I can tell you now that its far, FAR too open. When you design a map layout, think of it as a series of spaces connected to each other, not a big space you need to fill with crap. Look at the official maps, or some of the more popular custom maps on this site. They simply don't have big spaces like this, because it doesn't play well. Damage falloff and limited weapon range on most classes (with the obvious exception of sniper) limits most TF2 combat to close/medium range. If you want your map to be fun to play, it has to have an interesting layout. I don't want to seem mean, but I honestly think you should scrap this entirely and make a map with a much more detailed, thought-out layout.
tl;dr Big spaces bad. Make map better