For future reference here are the pages (paperback version) and a summary with my thoughts from my very recent reread . I still have no idea which is the "BIG HINT" that Sanderson mentioned.
Pg. 353 - When Raodan finds the secret library on the way to throw the old Elantrian into the pool. A mural implies that the pool might be a portal. Galladon says "It's like the water was considered a gate of some sort". I'm not the first person to notice this and honestly this seems the most likely to be the BIG HINT. Elantrians wouldn't have magic once they go through the gate, so they couldn't come back (and maybe not even survive there (Shadesmar?) without magic.
Pg. 396 - This is what I mentioned previously. I assume the shapes are not literal, in the same way we see a clump of stars and say "that is a bear or a hunter". Like there aren't actually giant lines in the sky forming these shapes, just pin pricks of stars that make up the general shapes of Aons.
Pg. 576 - Dilaf and the monks use magic to teleport to Teod. It requires sacrificing a monk (so it's a net-loss magic like hemalurgy that requires a sacrifice) but the teleportation doesn't seem to care about distance it seemed to imply any distance would kill one monk. Later in the book a flashback shows us a monk being burned away to teleport a short distance. This could be another way to world hop.
Other pages - Also mentioned before by others, but Tia teleportation seems to not care about distance. The only thing that seems to matter is accuracy of calculations, so presumably you could use this on any world if you have a way to fuel it (stormlight, etc).
For my efforts in connecting Aons to other books, the Aon for Honor (shard on Roshar) is "MAI" and the Aon for Endowment (shard on Nalthis) is "ESHE". My thought process here is maybe the picture of those Aons somehow match up with something on the maps of those lands. Maybe some kind of TIA MAI rune would teleport you to Roshar without calculating distances? If you look at the "MAI" constellation in the sky of Sel is Roshar somewhere up there?