That will probably never happen. It would allow players to “practice” against the toughest bosses and make act exploration way easier which would cost Kabam $$$$. You used to be able to do that in the Betas but they’ve stopped that as well
Better to add a training mode where you can train vs a champ and add custom nodes Will allow you a bigger range of combinations and will not be boss specific so you can practice a certain node or champion