I'm thinking there's another NSO update at the very end of the month, but that it's more likely Mega Drive than N64. Nintendo did April and June 30th updates for Mega Drive last year, so there's every chance it happens again. An N64 game would be more interesting, to shake up the predictability of things, but it really does feel like they're doling out what N64 games they have left as gradually as they can. The base level rollout of GB/C titles also looks slow, presumably so that Nintendo can keep easily adding stuff to NSO for sometime to come.
With 5 confirmed N64 titles remaining right now, it could be that those 5 are it for 2023. More interesting is the GBA, which only has 3 more titles confirmed for the service (F Zero Maximum Velocity, Kirby & the Amazing Mirror, Golden Sun). It's possible that, Mega Drive and DLC additions aside, those 8 titles are the remaining Expansion Pack games for this year.
I would guess that's a 2024 title at this rate. There are really high profile things - especially Smash and Donkey Kong 64 - which should make it onto the service and which would generate some buzz whenever they do get confirmed. It'd be especially good to see that rounded out by Banjo Tooie and Diddy Kong Racing, and then some other third party titles (Mystical Ninja 64! Mischief Makers! Please?). But we're soon arriving at the thin end of the wedge for N64 without further licensing deals with other companies.