I'd get some good layout software, like the suggested WinTrack, and the slowly plan my own layout. Take your time, even if it takes years before you start building. Think about what you like to do most when using your model railroad. Make that functionality a central element. Think about what you like next most, and add that (just as a concept block, not as a detailed implementation), etc.
Eg
1) Industrial operations - so you'd want to place 2,3,4 industries around the room that made sense to moving freight from one to the other; suppliers to factories to shipping? Maybe a port at one point to drop off for shipping (with or without a functional transloading facility)
2) Passenger operations - you'd want to place 3-8 passenger stations (say 2 big, 2 medium, one small, etc) along a line/spur that the trains would go through, stop, allow for overtaking.
3) Watch trains run in the country - plan in the proverbial 'paradestrecke' - the parade route, a long run of straight and gently curving run through some landscape (with or without bridges, varying trees, etc) where one can sit and watch a long 7-12 car passenger train, or 15-20 car freight train run past. Include some form of this in any case.
4) Modeling towns - leave space around the stations/industry to put some towns in to model streets and goings-on. Consider adding operational street loops with car-system traffic; maybe a place where it crosses a guarded rail crossing.
5) Modeling cities - bigger buildings, some urban features if you have the space (even as half-relief ), maybe a tram/street-car.
6) Modeling a detailed station and operations area - with all the tracks and facilities, a detaild model of just a small bit of the world
7) Changing seasonal content - find a place that is a manageable size where you can have a standard board that drops in that contains scenery. If its near a town the better - then you can model/build changing content on it that you swap out over the years. Eg a christmas market, an ammusement fair in the summer, a zoo, a rock concert, just a farm field, a soccer pitch, etc. This feature will let you keep building model scenes long after the layout proper is done.
8) season changes - interest in dividing the room into seasons - winter at one end, through a spring section, to a summer then fall section? Allows snow and ice at one end and lush green at the other. Consider scenic divides via mountains to allow sharper weather transitions without it looking too odd.
Then plunk down the big bits on a track plan (train stations/stops, industry spurs/sites) and plan how the track will work between the regions. Re-arrange. Try things out. Sleep on the ideas, revisit weeks later. Look at lots of YouTube videos of other peoples layouts, to get inspiration or recognizing what you like.
And if space is tight, and your not a prototype seeking modeler, consider running shorter trains than prototype - nothing wrong with 3 car passenger trains lengths, if passenger traffic modeling is important to you but a 7-10 car platform would restrict you to just one station.
I find the planning to be huge fun; I'm in no great hurry to start cutting wood and screwing things to the wall. Right now I'm considering in my own plan of adding a 3rd middle layer in my 2 level layout - to provide a longer running space for a parade route and relocate and give more space between stops in towns.
