The Teseract - no joke
Loki Writer Teases Marvel Cameos In Tom Hiddleston Disney+ Show
Oh please don‘t do that again. Don‘t rise expectation too high...
I won‘t expect Mephisto this time. Or someone like Kang or Amoralischere...
Since they are travelling through time, it opens up the options for cameo. My bet is an Eternal.
edit.... my second bet is Frigga.