are you talking about combining specs or having a class with a few separate specs? cos being able to choose holy priest OR demon hunter wouldnt make me a holy demon hunter, it would just make me ... well someone who can do either...
if i could combine things, well.... heck youd have to revamp the entire system, but if i could create a new spec by combining some things, yeah i think arcane warrior would be awesome, like a spell knight. maybe a priest and a monk so i could be more of a traditional monk instead of a pandaren trained monk (still salty about how panda infused that class is), shadow priest and paladin for something like a form swapping duality knight (cecil harvey from dissidia/ff4)
Probably the best ones though would be any caster spec crossed with a hunter since archer options in this game are severely limited by blizzard insisting that hunters should be the ONLY class that uses bows/guns and thats their identity, when really their identity is tamable pets. Share the bows blizz! i want dark rangers, shadow hunters, PotMs and as many archer options as melee options! hunters cannot and should not be all or any of those things!