Assuming they do have a tainted character. They seem to have their hands full without adding 3 new characters(including ch.3 one as well) atop of everything.

(But yes, that sounds pretty fun!)

ghost charon should take hearts or pickups at the beginning of floor (a toll maybe?) or at least like a trade for his help for the current floor at the start of each floor to make him a bit less op but otherwise great ideas

If you already do, then how much has been done in %?

My idea about Tainted Dante:
How about Charon would lead Dante to a special room where another unlocked character would be trapped by some kind of special monster (Maybe Exorcists from Mausoleum?). Defeating it/them would free the character and Dante can take them similarly as a companion (similar to normal Charon). The character would have random items (amount based on the floor so the number of bosses and treasure room, maybe some occasional Angel/Devil room item, Planetarium etc.). The hard part is that they will be low on health. It can be restored by picking up needed hearts (like Blue Baby would need soul hearts or the character could just have no heart containers due to items they picked). The HP would be shown similar to Strawman item or Soul of the Forgotten.When the companion character dies, then Dante will become scared for some seconds (the effect of skulls from Mausoleum) and have decreased stats for the rest of the room.

I know it may sound a bit confusing, so let me summarize a bit:
Imagine a character that you have unlocked playing run normally, the go through treasure rooms bosses, maybe shops and other rooms with items, but suddenly they found a special enemy which beaten them almost to death and trapped. Here comes Tainted Dante with Ghost of Charon. The ghost of Charon knows where they are trapped and leads Dante to help them. Dante finds finds them and (presumably) defeats that enemy and frees the trapped character. As a thanks to aiding them the character decides to be a companion for Dante. They still have low HP but it can be resorted by respective pickups.

Also the spacebar item of the compacion character (if they have any) would be in card/pill slot as many Tainted characters have.

Tainted Bertran could use his arms as he uses his head but instead of swinging use a punch like attack