Lore wise ANYTHING can be a demon hunter as long as they are intelligent enough and have another demon hunter teach them.
however the vast majority of demon hunters are either night elves blood elves or humans but its definitely possible so for gameplay reasons demon hunter could be available to all races.
now if we look at each expansion you will notice that whatever they add is always something fans want ALOT blood elves? lots of people wanted them. death knights? lots of people wanted them. goblins? lots of people wanted them. pandaren and brewmasters? lots of people wanted them.
so if we go and look at what gets requested alot the top three right now would probably be demon hunters, naga, ogres. its very likely we will get atleast one of those sometime next xpansion and considering its pointing to a burning legion expansion demon hunter is pretty much a done deal.
some other popular options are ethereal, battle mages, dragonkin or drakonid either as a class or a race, high elves, arrakoa.
every expansion theyve added something fans have asked for alot so we should keep our speculation to these