I read a thread that suggested that if you make a second WoW account on your Battlenet profile and buy the BfA upgrade for it, the lvl 110 boost would be applied to the Battlenet account rather than the specific WoW one. It's a thought. I'd quite fancy trying out a Rogue or a Priest to see how they play now. I've previously played both up to about level 20, but not since Cataclysm (the Priest I last played in vanilla wow.) You can pretty much level a character up to 110 in a week or two (obviously there are people who can do it much faster than that) but £16 to instantly get to that level without having to spend hours and hours in content I've done so many times in sort of appealing. I could boost one of the two classes, then leave the 120 boost you get with the next expansion for the other one. They're the only two classes I haven't played up to about lvl 50 or so.
Basically, since I've returned to WoW I've settled into life as an altoholic. I still don't have a level 120, but I do have five or six lvl 100+ and another few 80+ and I switch from one to the other, rarely playing the same character on two logins in a row. I used my lvl 110 boost to start a Paladin and boost it up as I'd never played one before, and going through BfA on that has been a lot of fun. Classes these days seem to be locked in to a very specific playstyle at about level 50 and you spend the rest of your time pressing the same buttons over and over, so playing a new class in a new expansion was really refreshing.
I dunno, if it didn't work I'd be annoyed. Even if I did I'd castigate myself for spending money on such pointless frivolities. I'll let you know what happens when I inevitably do buy it.