I would have to say that the scene in which we see Quaritch take complete control of the operation and dismiss Parker's threats would've been a great addition to the collector's cut as it shows the true colours of Quaritch and is a great moment of "redemption" for Parker.
Also, the scene were Neytiri appears to be pregnant with Jake's child would've given the viewers a good feeling before the scene with Mo'at and Neytiri along with the rest of the Omaticaya attempting to give Jake a new life as one of The People.
I loved the Dream Hunt and it would've added more forshadowing of Jake becoming the legendary Toruk Makto and in my opinion, this was the best scene out of all of them.
The festival was a fun scene and seeing Tsu'tey praise Jake for his bravery was a wonderful moment, even if it was ruined by Neytiri's dragging Jake away for a dance.
There's a great bunch of scenes to choose from but I only think a handfull at the very least would've made for good additions to the collector's cut.