I'm running into this now. Even though in real world construction there's only "new" and "existing", we STILL need to show what's to come.
Generally Existing is gray and either hidden or solid, and it's light...
New is darker, bolder, and solid...
Future is Red, normally a phantom (Double Dash, whatever) and just one weight darker than Existing stuff.
It's normally required because if you're designing, let's say a pump station, and by the time Phase 3 is completed, you may have 10 pumps installed. Five on Phase 1, three on Phase 2, two more on Phase 3.
If you start doing Phase 1 work without showing, at the least, the future equipment for Phases 2 and 3, someone is going to occupy that area with something.
I would even be ok with using the "Temporary" element in lieu of "Future" if that's a possibility.