As sort of a newbie collector still getting my bearings, I think my only really time-sensitive goal at present is to snatch up that "Friends of Equestria collection" set as soon as that's released, since it feels like the best starter for the Mane Six, has the nicest looking variants for the Princesses... But most importantly? G4.5 MINTY! And I'd hate to be left at the mercy of eBay scalpers if I could help it, lol.
But besides that? I'd just love to find more ponies in flea markets or yard sales in general (rather than over-relying upon online shopping) - and I'd especially love to start off the G1 branch of my collection, possibly with one (or more!) of the Twinkle-Eyed Ponies... I just love their whole aesthetic so much, especially Masquerade, Sweet Stuff, & Whizzer in particular!