Robbie won't be joining the Champions in the near future, because he's an Avenger. It's not a downgrade to go to the Champs though - there's already been six ex-Avengers on the team (Ms. Marvel, Spider-Man, Nova and Wasp from Waid's Avengers, Amadeus Cho from Dark Reign era Mighty Avengers, and Power Man from Al Ewing's Mighty and New Avengers), and when Aaron's run ends and the new roster inevitably doesn't include him, he could come over then, or join the Young Avengers if that team gets revived. It'll be a year or two before Aaron's Avengers ends though.
The team's already crowded anyway. Between Zub's Champions #10 and Outlawed #1, three members apparently left (they weren't in Outlawed) - Amadeus (moved to Agents of Atlas), Dust (went back to the X-Men), and Patriot (they stopped pushing him because Marvel Rising has ended and they can use the previous Patriot again). That still leaves 12 members, though most of them are currently dispersed. Current status:
Ms. Marvel - incapacitated but will be back before Champions #1
Spider-Man - active
Nova - active
Viv Vision - incapacitated
Ironheart - inactive (confirmed by her Iron Man 2020 tie-in, which should've been released after Champions #2)
Wasp - status unknown
Snowguard - arrested
Locust - arrested
Falcon - status unknown
Bombshell - status unknown
Power Man - status unknown
Pinpoint - status unknown
Clearly, Champions #1 will probably answer those unknowns. There's probably more that have been arrested and members who have quit because of the law.
And then there's the Reinforcement Roster Zub introduced but never properly got to use...
Honey Badger - now called Scout, she's with the X-Men
Silk - actually doesn't fall foul of Kamala's Law (she's Peter Parker's age, and probably the second oldest Champion after Cyclops), but unlikely to want to risk arrest for aiding underage vigilantes. Also, she's an Agent of Atlas anyway, so probably walked when Amadeus did.
Prodigy - we never knew which one Zub meant this to be. One of them is an X-Man though and is now with X-Factor.
Moon Girl - either arrested or on the run. We saw CRADLE raid her home in Outlawed.
Cloud 9 - considering her ending when Dark Reign ended, highly likely to have gone back into retirement as she did then.
Red Dagger - status unknown. Saladin Ahmed hasn't been using him in Ms. Marvel, where he originates from.
Hummingbird - the only Reinforcement that did appear (in issue 6, apparently having just been recruited before War of the Realms, hence why she wasn't in Viv's original list), however, she's a former New Warrior, and the Warriors sided with the law (of course so is Nova, so Sam should be feeling betrayed, will be interesting to see New Warriors #3 which guest stars the Champs, for that reason alone).
Cyclops - the X-Men should be his top priority, but it does sound like he'll be helping out considering the solicit for issue 3 mentions them turning to "the one adult they know they can still trust".
So what we need to know is, how many of these kids are out of action, and does that leave space for new members?