Ashley Williams/Kaidan Alenko: former veteran of the Alliance; current human Spectre
Garrus Vakarian: former turian soldier and C-Sec investigator; recently inducted Spectre/wanted criminal (depending on choices in previous games)
Tali‘Zorah: expert on the geth and electronics (given her fan-base (which is scary, to say the least...) and her obvious involvement in the past two games, it only makes sense that she be a squad member in ME3. Same goes for Garrus, only minus the fan-base comment.)
Liara T’Soni: former scientist and seasoned biotic
Miranda Lawson: former/current Cerberus operative
Jacob Taylor: former Alliance soldier; former/current Cerberus operative (not one of my favourite characters, but it doesn't exactly make sense for him and Miranda to be absent from the Normandy after ME2)
Urdnot Grunt: bio-engineered krogan super-soldier
Mordin Solus: brilliant salarian scientist; former STG agent
*Thel‘Jiral: prodigal quarian engineer/soldier; infamous Machiavellian-reasoning; one of the fiercest close-quarters combatants in the galaxy (utilizing a forearm-mounted shotgun and, on occassion, a machete); unique among the quarians (his armour would be like Jack's tattoos, in the sense that every individual piece tells a story about a part of his life, and because of a few dozen past experiments and genetic enhancements, his immune system is exceptionally strong for a member of his species)
Shiala/Aria: former asari commando
*Marqin Antovius: former turian soldier, marine and C-Sec officer; current infiltration agent (spy) and Spec-Op for the Citadel Council; one of the greatest strategists and warriors at the Council's disposal
Captain Kirrahe/Commander Rentola: veteran STG shock trooper
* = characters I've thought up in my spare time. I know there's no way in hell they'd make it into ME3 but it's a forum post so who cares?
/ = or, depending on choices in the past games
The way I see it, the squad is fair in terms of build (four ME1 squadmates, four ME2 squadmates, four new squadmates) plus it opens up some potential for good squad banter (Thel correcting Tali's work and the two generally getting on each other's nerves, Marqin talking to Ashley about his involvement in the turian annex of Shanxi [because everybody knows a "Yeah, I'm part of the reason your family is hated..." conversation would be epic], Kirrahe reminiscing with Mordin about their past missions, etc.)