I can't see Michelle since she really has no point to fight anymore, but Julia I know will return. Her redesign in T7 and her popularity definitely showed Harada she was a mainstay that needed to return.
I can see her making the base roster since she's a vet, Lidia and Kuni I definitely know will be base too since they were popular and Anna too, they'll have room since Noctis, Akuma(likely), Geese, and Negan won't likely return and I think all the current fighters are past the guest characters phase from years ago. The way I see it, if SF isn't gonna make a guest rep from Tekken, Tekken doesn't need to either. Guests imo should be for DLC rather than in the cast so we can have more veterans return
You can repurpose a character for refinement. Look at Street Fighter V did with Karin and R. Mika. Karin was mentioned before returning and appeared in spin-offs. R. Mika was left out completely since her only appearance was Alpha 3. Hell, Soul Calibur 6 did it with bringing back Hwang out of "retirement" when everyone thought Yun-seong was coming. Although Hwang did appeared as an NPC in Mi-na's story, there was about 25% chance of him returning as a playable character in the fans' mindsets and lo and behold, he returned.
The reason we need to bring Michelle back to the canon because Harada didn't provide any info on what had happened to her after Tekken 3 or confirmed if Julia's ending in 3 is indeed canon to be even declared Michelle officially retired of fighting/participating further the King of Iron Fist tournaments, unlike the other off-springs' stories like King II and recently, Kunimitsu (which had pissed off some of her fans). Even though Julia could or could not (God I hope not because if there's no Chang appearing on the base roster
again...) appear again to stay or further refine a character (her Jaycee alter ego now canonized from Tag 2), there is a chance for Michelle's return. What if Julia suddenly get kidnapped or held hostage again (like in Tekken 6's Scenario Campaign mode as she, along with Lei and Zafina, whom weren't Lars' opponents)? That could ignite Michelle's purpose to return. And I know that would spark Ganryu's interests... again for comedic scenarios.
As for guests, we may get one, as long they actually fit, unlike Noctis and Negan. I know Harada did want a Virtua Fighter guest (Lau) at the time.