At this point I think the scripting-engine should be plugable, meaning that depending what 'game' you are playing you would get another 'scripting-engine'. I don't see the point in making one-size-fits-all here.
Sorry, I forgot to answer this; yes I agree - as I envision it a more general purpose loader detects whether the data assets belong to Morrowind, Oblivion or Skyrim and thus loads loads all the necessary components for the respective game (all within the same openmw.exe binary).
The other option would be to have
OpenMW Launcher
|-Openmw.exe
|-Openob.exe
|-...
I am not sure which one is better from a architectural perspective, I prefer the first one but the latter option is likely easier to manage.
Edit: It is a bit worrying that
https://github.com/ponyrider0/openmw , c99cii , me (and who knows who else) are all working on different parts of TES4 without coordination.