Yeah a couple of things...
One, Cari already made a comment here ruling out cameos of the OIAL cast (though he didn't rule out references).
Likely explanation as to why is that importing model data from HS1 (the software OIAL uses for renders) to HS2 (the software Eternum uses for renders) can't be done, so Cari will have to recreate everyone from scratch in HS2 (which is likely a tedious process exacerbated by the fact that he has to significantly age them up because of the second point stated below, so he likely decided to just not bother).
Second, someone on Cari's Discord created a timeline based on the information provided in the game, stating that Eternum takes place 12-14 years after OIAL. Cari himself approved of this timeline and changed some things in the script to reflect it (such as changing the time Ulysses showed up from "a few years ago" to "more than a decade").