Its a safe bet, that if they had to bring in a whole new power line for Wicked Twister, they also put in some extra conduits to handle additional power in the future as the park expanded its need for electricity.
when is it deemed that a ride needs its own power supply? Likely when that ride's electrical requirements push the park's current electrical grid above its capacity. Cedar Pointe has a definable amount of power available to the park that is limited by the number of wires brought into the park, the size of those wires, and the size of the transformers and circuit breakers inside the park. Think about your house and the circuit breaker you have. There is nothing that prevents you from plugging in a hairdryer into every outlet, turn on the dryer and AC all while watching TV and leaving the fridge open. However, the electrical system in most houses is not capable of handling all that load at once, which will cause a spike in demand of current and trip the circuit breaker. The circuit breaker is simply nothing more than a safety device that prevents too much power from coming into the house or any one circruit so that the lines w/n the circruit and/or house don't exceed their capacity and start a fire. Now if you wanted to run all that stuff at once, you could certainly hire an electrician to bring a new line to your house, run new wires in the walls to different parts of the building which will then allow you to run everything at once.
That is essentially what Cedar Point did, they're grid was likely reaching peak capacity and Wicked Twister was going to push it over the edge, or close enough that fear of a power shut down from too many operations going on at the same time was a possibility. In order to increase their capacity, they went and brought a new line which gave them additional capacity for the park. Perhaps that line was sized specifically for Wicked Twister, perhaps that line had extra capacity for more park expansion in the future.
I'd imagine the location of a new attraction is a factor, though it may be a small factor. Keep in mind, not all new rides need a ton of power in relation to the rest of the park. I'm sure the motors on Diamondback have a pretty high hP rating, but its likely a small fraction of the amount of power needed to cool down a space like Flight of Fear and all the AC that building uses as well as the power needed to run the ride. Dinosaurs Alive likely uses very little power in relation to other rides.
the current infrastructure of a park likely pays a role in a ride's location. Its not overly difficult to run new electrical lines and conduit, but if you're going far off the beaten path that means you've got to dig trenches, perhaps saw cut concrete, patch up any concrete, etc.... Like any other business, they're looking to save money wherever they can. In most cases, the cost of installing new conduit for new electric lines w/n the park is probably a small cost compared to the cost of the actual ride itself.
I'd also imagine all parks have a special arrangement w/ the local electric supplier. Kings Island might be the biggest user of power in warren county (I don't know that, I just can't think of anyone else that would come close). I'm sure they get a break in pricing, I'm sure Duke Energy is aware of their future expansion possibilities, etc... I'm sure King's Island already has expansion in mind the last time they updated their lines coming into the park. If they're starting to run out of power, they will consider expansion the next time they bring more power into the park.