Sure they could hire a well known streamer. Take for example Bellular. Woud he go work at Blizzard for "normal" pay and bonuses? 99% no.
But they could offer him a deal good enough to make it happen. For example:
- Very good money & pay a good sum for him to leave his current work for a limited time (so they can continue without him until he returns)
- Total control over WoW for 1 expansion cycle (with option to continue if successful), everything from story to systems/rewards, raids/dungeons, PvP, whatever... assuming he wants to stay true to WoW and not change it to a battle royale or something like that
Will Blizzard do that? Hell no chance, but they COULD if they really wanted...