Gokaiger was an interesting series and I know people are already speculating on how Saban Brands will adapt it into Power Rangers. There is a lot of argument over the use of past Sentai teams on the grounds that they weren't in PR. Some people say don't use them, others say you must because Bandai won't like it otherwise. Bandai after all makes its money from the toy sales.
I have two thoughts on the subject. Firstly that when they adapt it they can use the past Sentai very easily as long as they explain the benefits of each form. Then it becomes more a matter of Ben 10 then it does about the keys accessing old powers. They only thing that would need to be discussed is why they have access to past PR powers, which can also be explained away.
Now to the COE version. In my view I wouldn't make them a team of good guys. I would make them one of the pirate clans who actively hunt down Power Rangers. When they defeat them they steal their energy much like Basco, and use the powers for their future quests. Their powers mimic those of the Rangers they've defeated but can duplicate some of their abilities... technology cannot replicate the training the Ninja Rangers received from Ninjor or any weapon that existed outside of their original powers such as the Power Cannon.
To be honest Basco would be how I would portray Power Broker from John Chubb's After the Siege series. The Power Keys are the distilled power of the Rangers they represent. Though not perfect he would be trying to reverse engineer them to unlock the greater powers they hold.