I'll say late August, because then they will have another one right before LM3 launches. As for predictions, here you go:
Safest: Their stocks will change.
Safer than safe: New features of Luigi's Mansion 3 shown that we haven't seen before.
Safe: Animal Crossing update
Mayyyybe: They address the joycon drift. Also another Wii U port (wonderful 101, for example).
Unlikely: Advanced Wars Switch is now in development now that Fire Emblem is out.
Not gonna happen: A RPG open world adventure starring every major 1st party Nintendo character. Like Kingdom Hearts-Nintendo edition.