Hi!
I doubt that will happen, Ocean is not a fan of harem, a "throuple" will probably the upper limit. But he is covering the bases for the various endings of WIAB, so there are hints into many directions, If Willi ends up with xx or yy or zz that you can see continuation in Summer´s gone. Some hints are stronger, directing towards the probable "Dev canon ending". Ayua for instance is with 99% surety
I theorise that considering the personalities involved and current known events, Willi is most likely to have kids with Miru, Leia, Zoey and Daphne (there is so much talk about "never with Daphne", but I think there might be a situation pop up where Miru consents to it). Not all in one playthrough necessarily, but I think they are the girls who will be closest to Willi´s heart.