I am not sure if you are suggesting that power poles should have a limited power flow capacity based on their size or whether you are proposing that the player should have the option to place a special entity that connects two power networks, but only allows a certain amount of power to flow between these networks.
It would be nice if power poles had a limited power flow capacity, since it would require your power infrastructure to have a stronger backbone, i.e. large power poles. However, this would also require the game to simulate power flow in detail, instead of having simple power networks. I'm afraid that this would be a lot of work to program and also would cost a lot of CPU time to make all the calculations.
Also, certain things which work now would no longer work. For example, it is possible to determine the battery power of the whole power network by simply attaching an accumulator to the circuit network. This works because, as a rule, all accumulators have the same power level. However, this assumption would no longer be valid if power couldn't flow freely inside a power network.
However, if you are suggesting that power networks should stay as they are and you are merely suggesting that power networks should be able to interact with each other in a more controlled manner, then this would be feasible, in my opinion. There could be situations where it is useful to limit the amount of power flow between power networks.
For more information about power switches and how power networks work internally, see
Factorio Friday Facts #115.
Are you suggesting a power switch with the added functionality that it should, instead of fully connecting two power networks, keep them seperate and only allow a limited amount of power to flow between the power networks?