Screens! most important. I dont care if its hammer ones and looking in dev. (then again, if you just started it then i dont think there is alot of show).
A few suggestions:
Environment: Alpine / spring
Nice, setting it this early already gives the downside of not entirely planning on gameplay. In this stage it should be more like:
Outside (dustbowl, goldrush/badwater, badlands/2fort/steel like). This basicly says that you make an open/very open/narrow outside map. This features gameplay above the actual looks. Notice that with the badwater outside setup you can still pick between egypt, alpine and desert. The space you got for outside detailing in the end should decide the looks. If its very narrow desert can be very hard but if you got lots of room i recommend desert. Desert maps have a far better distant look.
Further i already would say that alpine isnt a good choise. Its the easiest layout to look good and therefor overused. And many players (im sure) would prefer the classic look by now. Especialy if you are able to give a retro look like dustbowl/goldrush/2fort or steel.
Progressive: Yes, forward spawns will be implemented
For a 4 capture payload map this is already a must. no need to mention it. And note pl_swiftwater... where it has a forward spawn it also uses shortcuts. You can later decide which is best. (and most recommended would be having a forward spawn for each cp in the map but locked off. Then later decide which are open. This will prevent you from having to do weird stuff to get a forward spawn in. And best of all, you can check the smoothness from that spawn into the map.
Progressive: Yes, forward spawns will be implemented - yes, 2 comments on this depending on how you implement it
Dynamic: Yes, routes will open up / doors will open in relation to specific checkpoints being passed
Again something badwater had. This isnt allways a must, as example you can look at repository. That map only has 1 door and that simply opens when the cart has to pass. thats all... and the reason for that is that snipers probably were overpowered earlier and there had to be a wall to stop that.
ONLY add such routes if its required. in badwater it was easy to take the wrong route. and it spreaded out players too fast. Thats the only reason why its needed. If you can do without that you actualy do better than they did.