They're starting to offer refunds for GGG vs Canelo II. It looks like it very well could get cancelled, and Canelo could be suspended for a few months.
If the fight does get cancelled, Golovkin still wants to fight on or around May 5. There are a few opponents being talked about, but the biggest one is Billy Joe Saunders.
The current middleweight title scene looks like this:
lineal - Vacant
WBC - Golovkin
WBA - Golovkin (Super champion)
IBF - Golovkin
WBO - Saunders
IBO - Golovkin
Ring - Alvarez
To be "undisputed" you just need the WBC, WBA, IBF, and WBO, so Golovkin vs Saunders would give us an undisputed champion. And Golovkin would still defend the IBO belt, and since Canelo would be suspended Golovkin vs Saunders would be #1 vs #2 (right now Saunders is #3), so it would be for the lineal belt as well.
The only thing that would be in question would be the Ring belt, which may be vacated but probably not since it's owned by Canelo's promoter Oscar De La Hoya.
Assuming Golovkin beats Saunders, the Canelo rematch would be rescheduled for later this year when Canelo's suspension ends.
Hopefully the big fight still happens May 5, though. It's going to be a billion dollar fight, and we all know boxing promoters/athletic commissions/sanctioning bodies have been known to let things like failed drug tests slide for large sums of money.