So far all the stories about Silas are those that were fed to Shane from Silas posing as his dead girlfriend. What if it is not Qetsia who killed Silas' lover but Silas himself?..If Silas is as bad as he is made out to be I see no reason why while doing expression he learnt that the price of immortality could have been kill the one you love most. Qetsia could have loved Silas and so failed to kill him in turn but maybe he planned it all along, killed his lover in the hope that he could raise the veil to bring her back..
Furthermore since Silas immortality was conditioned by taking a life, his mortality(upon taking the cure or being killed by the one who takes it) is conditioned by bringing a dead person back onto our plane ..only one. So we could definitely get a dead character back it would depend on who forces the cure or death upon Silas..If it's Elena we could get Jeremy back,If its Damon perhaps Alaric,Stefan> Lexie ,.Matt >Vicky , Elijah perhaps it could be a choice between any of his family members...I think Elijah wouldn't want such a deal but Rebeccah with her loving heart would and so we may have hope of Kol coming back.Depends how smart the writers are.If I were the writers I would put in such a caveat and have Kol return and by crossing from the true death he could become the most powerful vampire alive and an antagonist for season 5. Or what would be most interesting is if Silas' lover was a werewolf , so upon his death we get an resurrected werewolf who becomes immortal (having come back from beyond) on our hands..it would be the best way to end season 4 actually coz it would mean a female antagonist much more powerful than Esther and a real worry for all the mfg.