Here's a characater with a ton of potential, yet a fear that he will never be utilized. For one, his powers were dumbed down a bit; making him less of a threat and not as omnipotent as his 616 counterpart. The X-Men, now prepared if ever in a fight with him again, could easily take him out. There is far too much skill in the X-Men to be taken down by someone with only steath kind of powers. Another thing is, Vaughan made him mentally unhealthy. There does have to be a level of insanity in any villain, but Brian went as far as making it a vulnerability. That's not good in terms of Sinister's character. I don't think he has it in him to become something his 616 counterpart did, which is really a shame. What I do see in him is the ability to do a 360 flip and completely shake things up... SHIELD, the X-Men, Gambit, Rogue, Apocolypse, etc. It's all very much a possibility but there are a ton of things that are holding him back from being someone much greater than he is.
Also, with Apocolypse, as of now he is simply a voice in Sinister's head. If you've read Shakespheare which I'm willing to bet all of you have, you learn to not read too much into something; you need to accept what you've been told and not add stuff to the story. That's all we can do (sit patiently, that is) until a writer comes around and tells us differently.