While I am no where near complete, I see several options. Not sure what level of complete that you mean, but here's a few ideas that I am going to do myself. Add more trees, one can never have enough. Add more detail parts, trash cans, mailboxes, manhole covers, backyard clutter and animals. If you haven't already done so, add drivers and passengers to vehicles that are on the road. While I am adding lighting to many structures as I go along, there will inevitably be many more that I can do later. Upgrade some older portions of the layout with newer materials, i.e. replace lichen with ground foam if you started that far back, or simply give the lichen a dusting of foam. Weather your rolling stock and structures, dull-coat your vehicles. I have a (not yet completed) creek that I am going to add skinny-dippers to, in order to use up some of the "artists and models" part of a Preiser unpainted figures kit that I have. Improve your structures with added details like hanging baskets, clotheslines, shutters or a fresh coat of paint. I still have a backlog of freight cars that need metal wheels (90 or so out of 800). Most of all, RUN TRAINS!
Hope that this gives you some ideas.Willie