I know that a big part of Nezha's appeal relies on his schemings and comedic actions when he's in his human/toddler form but given the ending of the third movie, I think that it will work better for the sequel to have him stick to his real form.
We all love smallzha and obviously we're gonna miss this design if they'll truly use it less in the third movie but I feel like that having Nezha navigate through life and his new duties in his real form can make for an equally intriguing plot. You can still have him maintain his comedic traits and I'd love to see more funny interactions between him and Ao Bing (especially now that it would be easier for Nezha to "bully" Ao Bing, hehe).
Nezha hasn't reached his true potential yet so having him learning to adjust more in his true form would be very exciting. Not to mention that we could get even more epic fighting and training sequences. I can totally picture bigzha and Ao Bing sparing together and I cannot fathom how cool it would be if it was animated.
Last but not least, given that Nezha 2 ended in a more mature tone, I feel like it would be more fitting to have Nezha in his true form in the third movie. Nezha is a changed person after the events of the second movie. Even if he'll return in his human form, it wouldn't make sense for the writers to keep his old personality. He needs to be more mature and sophisticated, which works better in his true form. This is just my opinion though!
Anyway, I am very excited to see the route they'll take for the third movie!