Some pets I wouldn't mind them bringing back or re-enabling a way of getting them because the first method was dubious to begin with.
Spirit of Competition comes to mind and I was really disappointed that they didn't redo that for the London Olympics. I can't see how anyone could feel hard done by if the pet was only obtainable once every four years in a computer game. The lack of having one isn't a reflection of lack of effort or time spent either - it was down to sheer dumb luck from the WoW Gods. That I don't like.
Another is Vampiric Batling. Now some (or many) may disagree with me on this one but hear me out. I was around in BC. I was also of level for Kara AND I was around for the Scourge invasion (I still wear my Tabard with pride). I'm not sure many people remember though what raiding was like back in those days (well, at least on my server) and especially for *oooo special sparkly bosses* like Mirkblood. Getting a run was virtually impossible if you were not already in an established raid team. They were insular, elitist, self-serving (so nothing's changed lol) and I know for my part, I never got a look in no matter how hard I tried. For our Guild, Kara raiding actually caused a split during that period, that's how intense it was. If they don't want to reintroduce the pet, at least allow it to be sold. Half those people don't play anymore or don't care about pets and would happily give it up for gold.