Personally, I would the emotes to focus on taunts... and of course other classy things like having your Ork character break wind. /K
I myself would like to see such things that others mentioned here, mostly taunts and to have a chance to laugh on my enemy.
I would be happy with battle taunts striking fear into the hearts of enemies, pulling out banner of your chapter, battlefield commands etc. and some simple emotes. Maybe some victorious shriek after obliterating your enemy, just not the dance please.
I think Orks should get the option to dance as one of their emotes. Seems like something they might do. I'm thinking the Charleston, personally.
After opening first charleston video on youtube and imagining orkz doing it.... let's say that I laughed more than I wanted. ^^
I want my Space Marine to dance J/K (kind of). Making in lore gestures like the Aquila symbol would be pretty awesome though.