Let me tell you something, what pushed Kane back WASN'T him losing his mask to HHH in 2003, he was still booked as a monster after that, he destroyed RVD, Shane O'Mac, and intimidated Goldberg and Evolution. The moment however, they booked him as the Undertaker's pussy, made him job to 'Taker in 6 minutes at 'Mania is what made people don't take him seriously after that.
Add that to the list of endless, pointless feuds he entered with, the storyline love triangle between him, Matt Hardy and Lita and then Lita having an abortion thanks to Gene Snitsky and then fighting Viscera and Trish Stratus made Kane look like a mid carder at best. They made him look pathetic in and out of the ring, and began putting him in lame storylines, once he began losing cleanly, he went from a monster to just another guy, FFS the guy danced with Santino Marella in the ring at WrestleMania.
Some might have a case that Kane always had a funny side to him even when he had the mask on, teaming with X-Pac and after that Hurricane Helms which is a valid point, but at least you saw the guy destroying everybody once he entered the ring, right now he gets his ass kicked by everyone which makes him look weak. He went from a guy that took three tombstones by the Undertaker at WrestleMania 14 to be beaten to a guy who loses cleanly in 10 minutes to Gene Snitscky.
If you wanna make Kane stronger, don't let him wear the mask again, no point in doing so unless you want to sell some 15 masks per show which isn't that much of a profit anyway, if you really wanna make Kane the unstoppable big red machine again, make him
kick people's asses in the ring, don't make him talk much and let the psycho Kane we saw at the middle of 2003 return.