I don't think there will be any sort of cryo thing going on. I think they will just figure out some way for the reapers to get out of Dark Space. Maybe there will be some Mass Effect thing (not sure what the machines are that they use to teleport around) sitting out there that they will reprogram to send them to a somewhere other then the Citadel. During this part of the game I bet you would go to each race Turian, Krogan, Salarian, Human, ect and try and convince them to join you. So maybe you could fail, and the race will no longer help you in the final part of the game. I think Rachni will only help if you saved the Queen in the first one, so choices like that from ME1 and ME2 could carry over. Then the final battle or final part of the game could be you doing a mini RTS game, where you sort of set up your army.
The other part of the game would be spent searching for a way to defeat the Reapers, or maybe a weapon that can actually hit the Reapers for some real damage, so the final battle actually feels like you are doing something. I really hope that it dosen't come down to press this switch to win, because that would be a pitiful ending. I would like to see multiple endings, for example one could be if you fail to get enough races to join you, and you manage the battle badly, then you lose, and the Galaxy dies.
I'd like to see a mix of new party members and old ones from ME1 and ME2. Mostly I'd like to see Garrus, Tali, Mordin, Legion, and Wrex return.
My main hope is that the game dosen't end with the simple flick of a switch to kill all the Reapers. I would be ok with by flicking the switch you caused a star to explode, or something along those lines that destroyed the Reaper fleet. I remember some sort of sun issue on the mission in ME2 where you find Tali, so that could come into play. I wouldn't mind you getting some sort of really powerfull weapon that could take down Reapers, so your fleet takes on the Reaper fleet.