Same here. Assuming legend slot isn't being considered a "headline" slot by Emily (sounds like that's Shania), and looking at other big female acts about, I guess some possibilities other than SZA could be:
Stevie Nicks (she's playing Cardiff with Billy Joel the week before, so maybe a co-headline set to top the bill?)
Kylie (doing BST but has did Legend slot in 2019 so doesn't seem that inspired)
Madge (somehow they've managed to agree something after the breakdown in negotiations)
Adele (her Munich shows don't appear to be an exclusive, just the venue they are creating is exclusively for her)
Rihanna (just played a wedding in India and was meant to headline last year)
Don't think any of the above are too farfetched, but there seems to be reliable rumblings on SZA so that's the best we got to go off until the big reveal later this week...