Could be either a regular upgrade of a very high tier (maybe passed 200 time machines) or a heavenly upgrade branching off from Starter Kitchen.
This upgrade would give your Time Machines a boost in CpS as a percentage of the amount of cookies you forfeited in the past, as you bring cookies from your own past back into the present. I think it would have to be a fairly small percentage though (Perhaps around 0.0001% of your total forfeited cookies, per time machine. For example if you had 250 time machines, this would give you a boost in CpS equal to 0.025% of your total forfeited cookies.
I'm still not sure if this would be completed balanced or not as is, but I'm sure the numbers could be tweaked a bit to make it fair, or make it cost a very large amount of cookies to get.