**WARNING: WALL OF TEXT**
I know these aren't suggestions but this seemed like the best place to post this, mainly because I'm tired of seeing all the suggestions for squadmates/worlds that are already guarenteed to return/make an appearance. Anyway, these are some of my predictions about ME3 -based on in-game dialog and events- and stuff already confirmed by BioWare (don't ask me for my sources of the stuff confirmed because, for the life of me, I can't remember since it's been months since I read the stuff and I'm way too lazy to go find them again):
Plot/ Worlds:
*There's going to be a 2-3 year gap between ME3 and the end of ME2. Makes the game "more realistic" since it takes 2-3 years to create it.
*ME3 will have a storyline similar to DA: O, a main character essentially going from place to place recruiting armies to help him/her fight a common enemy (the Reapers).
*Earth will be visited, likely to recruit the Systems Alliance to the war (BioWare already confirmed this...in a sense. Someone said they were saving Earth for just the right time and considering that ME3 will be the end of the series, it's now or never).
*The quarian homeworld will be visited, either to recruit the geth or reclaim it for the quarians (maybe both. Based on Legion's dialog in ME2, it seems like the geth would just give them back the world if the quarians apologized...)
*Turian homeworld will be visited, likely to recruit the Turian Hierarchy (again, this was already confirmed by BioWare in some interview).
*Tuchanka will be visited again and the krogan will be recruited (depending on player's choices on Mordin's loyalty quest in ME2 and the fate of Wrex in ME1. On top of that, the Illusive Man writes that the krogan would make great shock troopers against the Reaper fleet in the Mission Complete scene after Mordin’s loyalty mission.)
*Citadel will be visited to recruit the Council forces to the war (which will probably only be possible for players who regained their Spectre status in ME2).
*If the asari have a strong military force outside of the Council’s fleet, then Shepard will also head to their homeworld (don’t know the name) to recruit them as well.
*Omega will be visited to recruit the forces of the Terminus Systems under Aria (this is a personal prediction; there has to be a reason there were so many quests revolving around Aria and keeping her in control of Omega in ME2).
*Cerberus will return as either an ally or an enemy, depending on the player's choice at the end of ME2.
*The Rachni will return as a recruitable faction. Confirmed by ME2's dialog with the asari agent on Illium.
Squad members/Recurring Characters:
*Miranda and Jacob will return as party members (considering that at the end of ME2, Miranda resigns from Cerberus and Jacob probably followed suite).
*Liara will be a party member (once again, already confirmed by BioWare). Her recruitment/personal mission will likely have to do with the Shadow Broker and killing him.
*As mentioned above, the Shadow Broker will be a significant faction (either helping the player or trying to kill them, based off of that encounter in ME1). On top of that, BioWare may finally reveal his identity (my money's on Conrad Verner, his wife or some true identity equally ridiculous).
*Ashley/Kaidan will return as a party member since BioWare already confirmed that ME1 LIs would play a role in the final game. Based on how Ashley receives a promotion between ME1 and ME2 (don't know about Kaidan, cause I always kill him), the surviving character may be promoted again or, if the Council was impressed by their involvement in defeating Saren and their undercover work on Horizon, they may even become the second human Spectre.
*Tali will be returning, either as a party member because she was exiled or because she's the Normandy's chief engineer or as an Admiral aboard the Migrant Fleet. In the latter of the two, how the player treated her in the last two games would probably affect whether or not Shepard can recruit the quarians to his war.
*Garrus will most likely be returning as a party member, either as a Spectre (if the player "taught" him to be good via Paragon options) or as a criminal (if the player "taught" him to be merciless via Renegade options; the “becoming what he hunted” angle does seem like something BioWare’s writers would do). I thought this was likely to happen because both of Garrus' personal quests in ME1 and ME2 have you hunting down some old grudge of his and telling him to spare their life or letting him kill them.
*Grunt will be a party member, considering that Shepard is his battlemaster at the end of ME2.
*Samara will likely make an appearance but not be recruitable, maybe while Shepard recruits the asari or heads into asari space. Her reaction to seeing Shepard will probably differ depending on Shepard's alignment in ME2. Chances are she will either help Shepard (if he/she was Paragon) or try to kill them (if he/she was Renegade).
*Thane won't be returning...cause he'll be dead...
*Mordin may or may not be returning. On one hand, he's the chief scientist of the Normandy and, if Shepard was Paragon, he holds the key to curing the genophage. On the other hand, he's 50 years old, and salarians don't usually live past 40, so...
*Joker will be returning.
*Kelly, Dr.Chakwas and the other Normandy crew will be returning if Shepard saved them in ME2.
*Legion will probably make an appearance in some way or form, but won't be a party member. If anything, it'll be the one to convince the rest of the geth to fight with Shepard.
Ending:
*BioWare has already stated that ME3 will be the end of Shepard's story arc, which means he'll probably die during the last stand against the Reapers, giving his life to save the galaxy (Paragon) or being wiped out by the Reapers along with the rest of the Milky Way (Renegade, or really, if the player made nothing but Renegade choices throughout the game series because you won't have allies to ward off the Reapers if you were an ass all the time). Or BioWare will pull a "Halo" and have the main character drifting in the middle of intergalactic space and sleeping in cryo until someone buys the rights to the series from them and makes a sequel. (lol)
*Liara/Ashley/Miranda romances will probably end with some stupid scene in the future where the LI is parenting Shepard's kid. Obviously Tali romances wouldn't end this way because creatures of two different species can't have a child. It's biologically impossible. Plus, based on that turian/quarian conversation on Illium, I'm pretty sure human fluids are acidic to quarians and vice versa. And Jack romances wouldn't end this way because...well...Jack would probably eat the baby, or tattoo it to death...Probably wouldn't get this ending if Shepard was a woman either, since Shepard would be dead and the male LIs can't give birth to kids. If they could, there would be something very, very wrong with BioWare.