ZDoom

No more need to use console command to select witch display should gzdoom being displayed.
Instead, the user could just go to options, vídeo options and select the display he wants to use

Good idea. Even though I use my primary display for gaming (so I usually don't have to change anything), this can be of use for other people.

That was actually an easy change , I even did the change by myself.
I wanted to make a better solution, because this way gzdoom doesn't know how many adapters are in, but, I belive 9 is enough for anyone

Something like this is going to need more than an entry in the menu. As far as I remember, vid_adapter doesn't have any effect until you restart (or use a command line to set it on startup).

Indeed, it's not perfect, the cvar "vid_adapter" only changes when you change the resolution or changes from windowed to fullscreen.
I think the way menudefs Works, there's no way to do that (unless there's a way to run complex codes like if("fullscreen" == "Yes") { // change from windowed then full screen} )
OR, the cvar call needs to be changed, when calling the cvar vid_adapter, check if it's fullscreen, if it's fullscreen, do something for refresh the selected window.
That second option is more complex, but I'll give a try if I can do that

EDIT: Example code here http://forum.zdoom.org/viewtopic.php?f=34&t=50182

I'd like to see this -- while true that vid_adapter in particular requires a restart, a "You must restart to apply these settings" message is pretty common in games and is better than having to use the console/config manually.

Though at that point it's a matter of MENUDEF feature implementation.

I was wondering, isn't it better to use Borderless Gaming (or some other borderless window tool) and use it when the game is windowed on the monitor of your choice? Unless you really want to use fullscreen mode, but I don't see a reason for that.