It would be nice if the stations (especially rail) could be slightly modular in design, with a variety of upgrades, with the purpose of tailoring each station to the locale it is set in, rather than bonuses. Adding platform canopies to individual platforms, footbridges etc., culminating with a grand station upgrade (one per map) along the lines of St. Pancras / Kings Cross or Grand Central. Bonuses could apply, but then stations begin to look alike (as often you upgrade all for the bonus).
I don’t begin to think it would be a simple affair, but laying the groundwork and including a few options on release, and mods / DLC can add the rest. Always more immersive to see that lovely detailed train enter a station worthy of its visit.