He was a very heavy drinker and addicted to purple drank back in 2012 when he was 20, so I guess he's always had problems with substance abuse and I don't think it was much of a secret that he took the break up with Ariana badly. Ariana's newly released album had a couple of songs that were about Mac and the breakup, whilst the rest of them were about her new fiancé and that probably messed him up a bit and this appears to be the result. It's tragic because he's so young, but given his history and personal life not a super shock.
As for rap artists next year the only one I'd suggest is Lil Wayne who keeps having trouble with purple drank despite it repeatedly giving him seizures, anyone else would just be a bit of a risky punt.