Isn't that available already? (No windows to test.) if I choose --engine=luatex, most binaries and scripts will still be downloaded ( including mpost, kpse*, dvips/dvipdfmx and knuth's font tools) [not windows specific] because they are stated in mtx-update. while in fact only luatex, context, and bibtex are needed.
That's already working (should be at least). no. most of the afms/pfbs/tfms/vfs will still be downloaded (only encoding-fontname.tfms will not be downloaded). In MKIV, opentype version is enough except math fonts. (at least 30M download files are unneeded)
We need to split fonts first. Yes, indeed. If a user only want to use LM fonts (or no fonts in minimals at all), the download size can be limited to less than 40M (30M if users use their own fonts like lucida bright) instead of 100M+.