I was a bit worried when it seemed to turn into a musical, and hope Ncuti Gatwa won't sing every week. I think I can handle one "musical episode", but that's it.
So, the first mystery raised is Ruby's mother. My first thought was, Ruby will somehow turn out to be her own mother (I've been thinking about Red Dwarf here, where similar happened). I except this will be one of the big mysteries that continues throughout Series 14.
Then there's the identity of Mrs. Flood - I've already seen speculation that she's the new master. She could even be the "boss" that was mentioned in the first of the anniversary specials. She certainly recognises a TARDIS - based on her having a name that sounds like a body of water, could she be some incarnation of River Song, or even some descendant of Amy Pond? I can't wait to find out, anywway.