You can ask Blizz to delete your WoW account from your battlenet in a self help area (I find it and edit in a sec) and then buy it all again.
Edit: Oh Puny got it. Anyway I would advise making a new Bnet account because I have done this clean slate thing 3 times now and let me tell you… it is never clean enough. You will never get the satisfaction and eventually you will regret losing EVERYTHING!
It’s nice and relieving at first and then you realise… you can’t earn everything again. It is also exhausting. I have 2 bnets now and one is inactive. I leave it to exist in case I get this clean slate OCD thing again. Just trust me on this I know the issue so well. Keep your account just in case. It shouldn’t bother you because it’s a separate account.
And it’s not silly to have a separate btag. I mean it is but we are silly people aren’t we? But it doesn’t matter because it quietens the chaos in our minds and that’s all that matters. Just… be careful. You can’t undo removing your account.
You can keep your existing Battle.net account for your Diablo, but just use your new Battle.net account fo WoW.
Deleting a WoW account does nothing useful, since nearly all your mounts, pets and achievements are linked to your Battle.net account.
And this is a good time to do it, since in a couple of weeks we expect you will only have to buy Shadowlands, which you would have done for your old WoW account anyway, so you will have no extra costs.
Losing the ability to start a new character as a NEW character was one of the great losses of modern WoW.
Make a new bnet account, using the same personal information as the original account (same person name => your name; maybe they require a different e-mail). Then move the Diablo license to this new account (they allow it if it’s the same person behind both bnet accounts). If all succeeds, then go ahead and buy WoW license on the new account. Then forget all about the old one (you could ask to get it deleted, especially if you live in Europe, because of the GDPR).
Which will then force you to buy the game regardless because now you A: cant access the game with the same account and B: Cant reactivate the License because they wont allow you to do so. The license will be lost.
I have 3 WoW accounts on my main battle.net account. So it should be possible. I have my main account as usual. And 2 starter editions I added a long time ago, back when every subscription key box granted a free 7 week wow trial. Never used the other two in years tho.