Any game he makes next would need to feature a canon romantic pairing, which is why SAO works so well. I don't think there are any other pairings which have the canon romance and the popularity of Kirito and Asuna.
In the top 100 most popular shows on MAL, only 4 shows other than SAO involve a protagonist getting a girlfriend. Kaguya-sama. Toradora. Bakemonogatari. Oregairu.
There are a few more, in fact To Love-ru and Infinite Stratos work well because the girls like the MC explicitly and its not hard to stablish NTR. But yeah hte more real the couple is the better the NTR. Also since you mentioned Kaguya i hope that isn't next, i would feel too sorry for Shirogane xD, dude already is a mess even with his happy ending lol
Toradora now mmmmm, intriguing, the fact Taiga canonicaly goes a away for an extended lenght of time would present an oportunity itself, the only real con is that is doubtfull Fujino can do Taiga
flatness justice.
Oh and i remembered another harem that is good for NTR: SEKIREI, the MC actually gets with the harem in real poligamy but somehow still remains too much of a walking pussy so its easy toe njoy seeing him get netorared, plus the alien/winged aspect of the whole series (and even how it ended) also present good ways for ntr i think.