This is a question that's getting harder for me to answer every year. The core of my collection is done, its been done for a couple of years now. That being said I don't think it will ever be completely done. There will always be other goodies to find.
In 2013 I got some long awaited things that have been goals for at least a year or more, like a Tandy Coco disk drive and controller, a CIB version of Mario XE, a couple of rarer CIB Atari 7800 games, NES vaus controller, power base converter, a nice CIB Atari 2600 with matching serial numbers, a Commodore 64, a Famicom to NES converter and some Famicom games, and a PS3.
My one goal from the last two years, which is still unfulfilled, is to find an atari 800 computer. I have 800XL's and a couple of XEGS's but the 400/800 line of computers will play some carts that the others won't (at least without a translator disk). Wouldn't mind a disk drive too.
Will also continue to work on my Commodore 64 collection. Have just a half dozen carts at the moment. Maybe a disk drive for it as well
Also some select Wii, PS3, and NES carts but nothing must have, just some nice to have games.
I don't buy "shelf filler" anymore. I'm long past that. Who knows what else 2014 may bring