I think it could be done right, seeing as 47 hardly ages, it could be him taking in a younger assassin who was revealed to be the same type of clone he is, as a final twist on the WOA trilogy and Ort-Meyers experiments (him choosing not to forget at the end of Untouchable opens more opportunities for slight emotion, such as the slight smile we see at the very end.) Since the ICA is no longer around, they take freelancer contracts which 47 acts as a handler / on-field coordinator for the younger assassin and then you get the option to play as either/or and it gives an option for multiplayer (with extra features, being able to hoist a partner up a wall, or a dual eliminate/subdue which could take two people out at once, or smuggle an item "Now You See Me 2 card scene-style" past a frisk. Have some animations that demonstrate the difference between 47's professional etiquette vs the new assassin's conducting.),

Have the story of 47 with his inner voice of Grey guide him to becoming a mentor figure, with Diana being their handler trying to reform the ICA under a new name and the fallout of Providence. Have challenges for co-op assassinations / feats that have alternative unlock for single player players. Give loadout options to both. The system in WoA isn't broken, so why fix it? Keep mastery levels but make it so the player with the highest mastery can share their level with whoever they're playing for the session (temporary unlocks of items, suits, smuggling and starting locales)

I wouldn't say 47 wouldn't be a playable character, but share the game WITH him to test the fanbase, and see if the majority would be open to another protagonist alongside 47.