Dipper: Alright, now that we've defeated Bill Cipher and the show's over, can we ask ANOTHER question about Gravity Falls?
Alex Hirch: Very Well.
Dipper: Okay, so here's what confuses me about-
Mabel: ARE BLUBS AND DURLAND A COUPLE????
(end credits)
^ Joke stolen from YGOTAS Episode 65

In all seriousness, I get the appeal behind leaving some stuff open for interpretation or speculation for us fans. There's more than enough questions about the world of GF or just general curiosities (Seriously what the heck is Dipper's name, and is it REALLY that bad?), but the important thing is that the finale itself wrapped up the main plot and gave you the most important answers, giving us a pretty solid wrap up.

For the most part.

I feel like there are some things that could have been addressed in the finale, but weren't due to time restraints... I was kinda hoping Hirch would mention something a bout an extended version, whether on Disney XD on a special night or on a DVD specially dedicated to the Weirdmageddon Arc. To make up for this, I've an idea for a oneshot featuring Dipper and Mabel taking place during the events of Take Back the Falls. Nothing big, just a conversation that happens to drege up a few things, like...
- The whole "I gave Bill the rift directly" thing
- Whether or not Dipper would truly be okay with sacrificing an apprenticeship, and how Ford would react to his decision to go home
- Dipper letting Mabel know about Gideon's sacrifice

And others if I can think of any. And maybe throw in a Macbeth reference if I can help it. (Don't ask)

What are some things left unresolved that you still wonder about, or hope gets addressed at some point in the future, whether through merchandise or something else? (Also any advice or suggestions for my oneshot idea?)

5072620 At the least, we have Journal #3 to look forward to, and that might elucidate some things, though it's unlikely to include what you mentioned, for obvious reasons.

5072620
Dipper's real name.
Where the alien prison pods went to.
Who the ancient power is.
The reason why gravity falls has such a high number of anomalies and creatures.
The weird brain/face in the freezer in the super market.
Where Robbie go this brain washing CD from.
Did Giffaany really die.
Where did Blendin go after Time Baby was killed and was time baby really killed.

More like answers left unquestioned. Ancient Prophesy of individual Destinies? The Gravity Falls Weirdness Magnetism? Pretty major cop-outs.