I only have a single garage, which currently has parts for two projects, a 138in wheelbase slingshot dragster, a small lathe, pillar drill, MIG, TIG, 150ltr compressor, engine hoist, two roll cabs and top boxes, 5ft work bench, vice, folder, shear and all power and handtools stored. I have to keep it tidy and really struggle with storage but keeping the floor clear really helps.
If I were building from scratch, the things I would do is separate car storage, parts storage, workspace and bench areas. Move the compressor out the garage to reduce noise, have a basic extractor set up for fumes, 32 and 16 AMP sockets, as much lighting and 13a sockets as possible, (every 5ft). IF you're serious about fabrication, three phase would be the next move to use more powerful machinery. A ramp would be nice but not essential in my book, takes up a lot of space which you might want to convert later for other uses ie, spraying, chassis jig, machinery or parking. A decent RSJ and hoist would be really useful, for engine work but also lifting off cabs / bodies etc. I wouldn't bother with a pit either, dangerous to use on your own
Having the right comforts helps too, heating a big space is expensive so think about how you do it efficiently. Bring natural light into the building wherever possible, decent sound system, kettle etc.