For me Future World is the place to go at Epcot. World Showcase already as the Illum Holiday Show, Candlelight, and the Holiday Storytellers. Future World has barely anything, and has all that available space in the evenings. Lights of Winter fitted there perfectly, and the whole area around Fountain of Nations, and the bridge to World Showcase could serve very well for a light show.I suppose you are right.
I don't see it working anywhere in Disney Springs very well, for a variety of reasons (tenant concerns being one of them)...but there's no reason why they couldn't work parts of them into an expanded, new and redesigned World Showcase holiday spectacular.
I do agree, Epcot (World Showcase in particular) seems an obvious choice...at least for certain pavilions (wouldn't really fit with Japan or China, for example....and maybe Morocco, but, other things can be substituted there).
However, I think we'll probably see a number of small offering split between MK and DHS, rather than a new blockbuster addition.