The current problem with Kane is that he has been (since his unmasking) fully humanized. When he first made his debut, he was fully covered (in that, no portion of his skin was showing), claiming that his face/body was burned in a fire. Due to the burns, if Kane ever wanted to speak, he had to use a speech device. However, months after teaming with X-Pac, he begins to talk without it.
Somewhere down the line, around 2001, he turned heel and, in turn, changed his attire. He wore what was basically a tank top version of his costume. He would wear that until 2003, when he would go on the injured list. Upon his return, his mask was altered and his mouth was now visible......and then came the "Kanearooni" (which, IMO, is basically what killed the character).
Fast forward to his feud with Triple H, with H accusing him of "murdering" Kane's own girlfriend. So, either the "burns" he suffered were not as bad as everyone made them out to be, or Kate Vick must've been blind. Anyways, the culmination was that Kane lost his mask.
After losing his mask, Kane goes back to being a monster heel, even going as far as tombstoning Linda McMahon and electrocuting Shane's testicles. The "monster" was then basically fed to Undertaker at WrestleMania. Then, after "impregnating" Lita, continues to go downhill.
Sure, he's shown signs of "monsterism" but, without the mask, Kane is just a human. Similar to how Taker wasn't really Taker until he "returned from the dead", Kane needs to be put in an angle that gets him back as the Kane we all know and love. Perhaps an inferno match angle where Kane loses after his face gets burned.....then comes back the mask.
I think he still deserves his shot in the spotlight, but I doubt it's going to happen because the top looks far too crowded.