Here are some very common suggestions:
Elevators We have those!
We have those!
I've heard rumours.... but not yet. Not as simple as they may seem, but not impossible.
Not yet, but some have tried. Not impossible, just not too likely right now
Ehh…. technically possible, but the vehicle system is not great, and is due a big update at some point. So maybe then? Best we can do right now is put a new body on an existing vehicle.
There have been mods, there are some now, and there are some in the works
We all want those! There is work in progress, and there was a mod which did it in a way, but the one in progress will work as you'd expect. Just not as simple as you might think... and Factorio is easier to do a blueprints mod for, so...
A very specific train
We get more requests for very specific modern world trains than you might think. While technically possible no modder has made one yet.
Kinda sorta... There is a mod for road themed foundations but nothing yet where you can build roads like you'd build rail
While you can add things (like rocks, trees etc...) it's not currently possible to make changes to the map itself.
A flat world
Mostly! You can share a SMM profile (essentially a mod list) via settings in the app or you can create a empty mod with the mods you want as a dependency.
Change resource nodes
We had that maybe it get's updated someday.
Pipes mk3 etc..
Kinda! While proper mk3 or faster pipes are literally impossible right now (game logic problems) there is a mod which mostly does it.
I don't want that mant a in my factory
We have a solution for that!
We have that!
Creative mode We have that!
Flying but not with PAK mod Yup!
Change time of day
Core mechanics from other games
Ride animals / wall running / flying / or whatever else.
While technically possible, most stuff like this requires a LOT of effort like animations, models, lots of code etc... Not so easy to 'just add X'
But there is one for Double Jumping
"can't you just do X?"
If it was that simple someone would have done it by now.