I still cannot see how this format would work , to be a TT series the races would like the TT have to be on roads . You own the most unique motorcycling event in the world with popularity on the up and you want to consider promoting a series using the TT brand which in my view might actually do it more harm than good.
If there are enough road courses out there to promote a series, then call it something else like the International road race championship . I searched this on the Web to find it already exists.

But even with the right name would it work ?