Cooper has spoken out about the rumours, denying them:
“I’ve heard absolutely nothing of this! It’s nice to be thought about though, wherever it came from. It’s a very nice acknowledgement. Whoever does take it on has a very very hard job. Matt has done such a wonderful, wonderful job with the intriguing, interesting and brave choices he made with him. They’re huge shoes to fill.”
“What an opportunity it would be.”Despite the constant rumours, the search for the Twelfth Doctor has barely begun, and we shouldn't be expecting an announcement for at least another month or two, when the Christmas Special, where the Twelfth Doctor will make his/her first appearance, starts filming.
[Source: Doctor Who TV]