I think that the result of this fantasy would look like the actual bout between Sergey Kovolov and Bernard Hopkins. Adonis Stevenson packed a punch like Kovolv. Stevenson by TKO 6.
I take Hopkins by close decision, could perhaps be by MD or SD. He still had a very good defense & good precision in his punches at that stage, but his lack of speed and punch volume would make it hard for him to win rounds decisively.
Hopkins by that point probably had enough to still decision Stevenson. It'd be extremely dangerous in a way that the Tarver fight wasn't. However, Hopkins stylistically is literally built to batter and school a guy like Stevenson.
If its " The Executioner" Hopkins then he wins. If its the "Alien" Hopkins then he might lose but will mostly leave it to the judges using the ten point must system.