Originally Posted by DenWatts:
“We need another time-travelling race to be able to defeat the Time Lords.”
The Daleks have had rudimentary time travel for quite a while, as have the Sontarens and the Cybermen.
The War Lords also had knock-off TARDISes (SIDRATS).
The Tharils of E-Space we "time sensitives" who could ride the time winds.
And let's not forget the Black & White Guardians of Time.
I'm sure there are more.
One of the self-styled roles of the Timelords was to prevent effective time travel by other races. It was always assumed that other races could not be trusted to obey the First Law of Time (which, interestingly became the Prime Directive of Time in Star Trek - and people accuse DW of being derivative!).
So, either a cartel of time-savvy races (led by the Daleks) have got their cranial cases together and found a way of transcending the time and relative dimension barrier, OR a rogue Timelord has leaked the secret, OR the Doctor was somehow duped into revealing the secret.
Either way the result was a Time War.
Many years ago I submitted a draft to Virgin Books for a Dr Who story. It was called
The Time Healer and it involved all (7 at the time) Doctors.
The basic premise was that, because of all the "illegal" time travel, a chronofracture had occurred. Time started leaking all over the place and chaos was ensuing.
The White Guardian calls on the Doctor once again to put things right.
The idea was to create a tidy time line for the Doctor's stories. It meant that some stories "never happened" and opened the door for a whole new set of adventures with a solid canon to work from.
Sadly the idea was rejected

(I'm sure a lot of wannabees were submitting at the time - I was just one of them!).
Perhaps part of the "reboot" that RTD has mentioned is to crate a similar situation. Yes, things will return to normal, but it will be a new normal - a template for future stories and a tidier mythos to deal with.
BTW
Den, I love the way you picked up on my concept and ran with it - sheer brilliance!