I was thinking/hoping it could be Fett as well. Think of the tension there'd be if Fett was traveling with Mando & Mando had no idea who it was. Fett just biding his time.
That said, I respectfully disagree that an episode needs to advance the overall story arc to be not a waste of time. I look to 2 of my favorite series, Samurai Jack, & AVATAR: The Last Airbender. Jack's story was that he was trying to get back to the past, but in 5 seasons, only a handful of episodes actually dealt with that mission. Most were just Jack living day to day, but those episodes let you learn who Jack was & ultimately made you care whether he actually did what he was trying to do.
With A:TLA, I've talked with people who followed some online list which supposedly let you know which episodes were considered 'filler'. Almost every time, when I talked them into watching those episodes, they ended up enjoying them & got a little more insight into who the characters were.
Just my POV.