First things first: you're the creator of the game, it'll always feel too.. "Empty" or "hollow" because you know everything about it.
That being said; adding goofy dialogue with the NPCs, side quests that can be redone and/or alternative goals can help.
I experienced this before; and for one game, I added a fishing mini game like thing. If you wanted any of the new post-game stuff, you'd have to ice-fish and make enough money to buy it. Also adding new characters to help the hero was a opportunity to make fun interactions with that area depending on each characters ' personality.
Another game was very empty at the end; so I added a completely new quest. A group of Super bosses appear and you have to defeat them all at least once; there's 3 you can grind for more experience, and it keeps things fun for a little longer.. The ultimate goal is to amass enough money and weapons for the final area...
But (like I said earlier) even doing this and adding a true final boss felt empty since I know what it would take to do this and what reward is at the end, even from the start of the game...
Make your game world fun, quirky and (unintentionally) mysterious, that's how people continue playing Mario and Zelda64 for decades after their original release and are obsessed with them!