I’d rather there not be squadmate spin-offs, but if there had to be one I’d go with Legion’s story leading up to his appearance in ME2.

I think playing as Liara as the shadow broker would be interesting (although I think this would make a better resource management sim than an action game) or Garrus in his lead up to joining the team seeing as he was a possibility for a specter. Wrex would also work since he is a very old krogan merc. I feel like a bunch of mini adventures for all the characters would be the best option for a spin off. Perhaps import your save and play through your Shepherd’s background.

Out of the options above? Garrus, 100%. However as an alternative I’d like to see Legion doing his thing looking for Shep on human worlds, sneaking around. But Garrus is probably the one I’d go for.

I'd love a "Miranda: The Early Years" spinoff.

Yes.

Either Liara, after becoming the shadow broker, or Miranda. There could be a good amount of expansion with Laira after taking on the role of the shadow broker in between ME 2 and 3. And for Miranda, she could have some intersting stories with her time at Cerberus, or alternatively, after she leaves them and goes on the run.

Garrus, thane, wrex I’ll even take a jack spin off game

What about multiple short stories with different squadmates? Thane assassinations, Kasumi Heists, Miranda operations, Wrex battles, Zaeed bounty hunting, Garrus Vigilante work, Tali ship infiltration, maybe a game that goes into detail about what the characters in ME2 were doing before the game that'd be cool you play as alot of different characters with different classes and it'd be somewhat linear compared to a mainline game since they aren't the playable characters and have grounded backstory already.

That’s an interesting idea, I’d add in samara and would be more interested in her life before and after taking her oath

That would be difficult to quantify since she took the oath 300 years before mass effect 2, maybe her finding the lead that brought her to Illium to find Morinth more recently