Yes, the GK position is a tough one. Klopp already tried an upgrade with Karius and it didn't go well.
Personally I don't think Klopp handled this well, benching Karius because of public pressure. The time to give him a run even in face of patchy performances was then, it's passed now, and Karius has looked shaky ever since.
(obviously I can't know for sure whether it was indeed the public criticism which led Klopp to take Karius out of the team, or bad training performances)
Mignolet had seemed to step up and he has been pretty great, mostly, but he has the uncanny trait of letting his team down badly in rare, but really decisive moments. The Arsenal game blunder was one such moment.
However, it's not easy or simple to get a clear upgrade just like that.
If Koeln get relegated (which they will), Timo Horn should be leaving in summer and Klopp might go for him. I'm not sure about suitable candidates from other leagues.