the launcher could be named spring.exe, and double clicking it would result in the exact same happening like double clicking spring.exe now. lobbies would not need to be changed at all after this change.
the only thing that would not be possible anymore is double clicking spring-dedicated.exe, spring-headless.exe or spring-dedicated-client.exe, but one had to start them through launcher.exe/spring.exe with the appropriate option (eg --headless). I hardly see this as a bad thing though, as it possibly can even help preventing confusion ("why do i only get a black window when starting the game? (double clicked on spring-headless.exe)").
I don't know how springie and spads work, whether they use the dll already, or rely on spring-dedicated.exe. They would need a change most likely, which would be about this:
replaceInSource("spring-dedicated.exe" with "spring.exe --dedicated")
this would be a very low-level/basic change, and does not at all touch something like the ingame menu. also, i never heard of anything like the start script dir, as you should allways give the full path to the start script, and it does not matter where the script lies.
totally forgot about MT version! yeah would be nice for that too, also there, we could make it use MT version by default on systems with certain specs (eg more then 1 core), if that is considered stable enough. (which as i got it, mainly depends on the widgets used though :/ )
launcher would be mandatory of course, think of it as the new spring.exe. its just that it relies on more dlls then (eg 24, instead of 20).
I did not yet think much about which parts should come with the installer, and if we should build in a downloader into the launcher to download the other parts when needed/requested, but these are things that get possible then.