I have several issue with Silas, first How can Silas speak English... he's ancient is it a result of his illusions? Honestly I'm not even sure if he is a doppleganger, they keep changing his character. (Bonnie can see him then she couldn't) I'm also curious who Silas's love was, I'm praying it wasn't Tatia's ancestor somehow.
Also, I believe Shane said that Quetziah (I know I'm spelling her name wrong) helped Silas create the Immortality spell, but then in the finally Silas claims to have done it alone.
I think they are just going to make Silas a pathological liar, I don't understand why he couldn't take the cure, go back to being a warlock and tear down the veil himself instead of relying on Bonnie. I mean he wants to take the cure and die anyways (Supposedly). I wonder, as his doppleganger is Stefan able to harm him?
As to what Silas has planned in Season 5, Bonnie is dead, so maybe he's just sticking around for Elena and Katherine to get back at them for using the cure. His expression triangle is gone, I'm not sure what his next step is.