Someone from the old generation would be more interesting and surprising but the most likely answer is Boruto. I can't see Mitsuki having a particularly normal ending in the series to be honest. Does domestic life suit him? I can imagine him having Ibiki's job but in the Hokage office seems strange. So yeah, i'm going to go with Ino.
Her Kage Summit guards are Boruto and Kiba.
I kind of want to rule Boruto in and out because if he wants to play that support role in Sasuke's sense, he might not always be in the Village. Even when Sasuke is in the Village, Shikamaru still plays his vital role alongside Naruto, so its not like Sasuke replaces him either.
So Boruto is either going to play Sasuke's role only to Sarada, or he'll play both Sasuke and Shikamaru's role.
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Flip side being Mitsuki, if he accommodates with domestic life like he's doing now, I don't see a reason for him not to help out. His whole purpose is being by Boruto's side, but at some point that's going to be near impossible. So I guess the next best thing is him finding his own purpose, which might be helping Sarada out. I mean it would make him more interesting than his cheer-leading thing he has going on IMO.
If not either, Shikadai by default. I would rather have an oldie though. Maybe Asuma's daughter?