My guess would have been:
* Bane- but he's human and the Shadow Broker has been around for a while, plus i'm sick of all the turians in the galaxy.
*Barla von- makes much more sense, expecially since he's a known information broker, but in a cut news cast he dies...
*Reaper- which would be Sovereign but then it dies in ME1... so did Harbringer take over... or is there another one... dun dun dun...
*TIM- altough I really hope it isn't, and it would just mean that he delibertly worked against himself...
*Misc. guesses: left over prothean, some new species, Blasto, and the ever infamous liara's dad and Alena (Or however you spell her name).
*Most likely candidate is an asairi, because of their age and wisdom, salarian (female?) because of their huge wealths of intelligence but it would have to be several on account of their life spans, or volus, because of their trade/ diplomacy skills.
I think my final answer is Barla von, since we've met him bevfore and it did seem a little obvious when we first met him, but hey, a surprise would be nice. Hmmm that makes me think, do we actually know if we will find out who the shadow broker is, I mean, Bioware could do another Tali's face reveal... or maybe we have to choose between capturing the shadow broker and finding his identity or rescuing the entire Normandy and her crew from a nuke... hehehehe
But yeah I know my guesses count for nothing as pretty much everything has been said but goodluck everyone