Free lvl 100
Have no idea what I want to play in legion horde or alliance, currently horde lmk

I think alliance will rule this xpac again, if you want to play fotm roll with a shaman, either elemental or enhancement.
Another option could be druid as well because you can do a little bit of everything with them.
Honestly if I were you, just think about what you would be interested in playing, then watch some game play videos on that class and decide if that class is for you or not, that will help you with sticking to that character and you'll have the most fun with it. :D

I'd say go rogue since they're soo bursty and good at controlling the fight. Depending on your gear, you can pretty much kill anything within a stun. It's nice to be able to pick your fights and peace out when you're too overwhelmed.