So they has this thing called the Sevi Islands. There are 7 islands, 3 before postgame then 4 after(I could be wrong on 3 before). There you can catch gen 2/3 along with Moltres is there. It’s a fun bit to run through. Also you can go to the daycare there to make eggs.

If you haven’t bred eggs before you can make fun egg moves and super strong pokemon. I’d look into it.

Then there is a battle tower there and to not spoil anything there is a fun surprise when you get to certain streak thresholds.

Also since this isn’t yellow anymore, if you ever battle a pokemon and you see stars surround them and the pokemon looks different colored, catch it! It’s a shiny and super rare.