I wouldn't be surprised if Gal Gadot got a cameo or guest appearance, along the lines of Ezra Miller on the CW last year, but I don't think we'll see Diana as part of the regular cast.
It's possible Diana could be included of course, her movie did well and she's well known so it'd be a benefit to the show, and HBO might have the budget to afford at least a minor role for her. Plus, we're getting cross-platform material with Disney+ and the MCU so WB/DC might try to mimic it. But I think a cameo is about as good as we're likely to get.