They definitely left it open for season 3 -- homeless Marcella coming back undercover. The only thing about that, they would have to move her to a completely different country because I think law enforcement would know in a heartbeat it was her. She walks like a little banty rooster, and her attitude in doing things, if she crosses paths with people from her area, they would know her I think.
She murdered her baby. She killed this kid. How are they not putting her in jail? Instead, they give her a job?
And her kid and his fascination with killing animals, did he see Marcella kill the baby? They didn't really solve that. And they never solved the murder of the band guy by his band mate. And they didn't solve Tim's case of embezzlement. And they didn't solve the rich couple's issues. And they didn't solve what the hell was going on with the husband and Marcella's memories of him hurting himself and framing her.
What else?