While it's true that Smash generally ends on a whimper, this doesn't take into account that Smash DLC before hasn't ended with an E3 reveal either.
If you're making a prediction based on the pattern that since Smash has always ended on low-key characters, we will end on a low-key character for FP2, then likewise by the same logic you could make a prediction based on the pattern that E3 has always had an at least somewhat exciting character, therefore an exciting character will be revealed at E3 2021.
With that said, I personally don't think we have 2 huge third parties left either. However I think it's a fair assumption that E3 will bring out at least one "Hype" character. Now hype is subjective, but Nintendo is at least somewhat aware of fan demand. There's a reason why Ridley was the E3 2018 reveal and not Incineroar. There's also a reason why Hero and Banjo were revealed at E3 2019 and not Terry and Byleth. Regardless of how you feel about these characters, I think it's undeniable Banjo had a much bigger impact on social media than Byleth.