Yea, I thought ME3 was pretty good ... then the end just seemed to ruin EVERYTHING!! But it may also (if intended as I hope) be the most brilliant thing ever.
If you've finished it, have a look at the official forums as spoliers wont matter much.
Theres some theories that seem to make sense if you think about it. And in some cases, was hinted at through the last couple games.
Listen closely to what the Rachni Queen has to say (if you saved her) for example.
I had thought about this after I replayed the end sequence, as the last 10 minutes really do not make much sense. Theres a lot of subtle things around if you pay close attention that just do not fit.
My take is that theres really only 1 real explanation .. but that means future DLC is a absolute must.
And, Im hoping thats the case... but it should be a free DLC. Charging for would be a bad move IMO.
((I know this probably seems vague but Im trying to avoid spoilers. Its no secret well over 90% of the ME fans dislike the way it ended, but you can be your own judge there I suppose))