Foreign S-Corp operating a nexus in another state
There are any number of factors which can establish nexus in a state. If you are physically operating your business from PA, then you certainly have nexus in PA and need to register your business there even if it is incorporated in Wyoming. Incorpoating in Wyoming is not likely to reduce your PA income tax. Also incorporating in Wyoming will mean you are subject to Wyoming tax. While Wyoming does not have an individual or corporate income tax it does levy sales tax. Simply expanding to another state will not necessarily subject you to nexus in that state, as you generally have to have some type of physical connection to the state such as employees, assets, owners or physical business operations.
Every situation is unique and nexus is very fact based, so I would highly recommend that you contact a tax professional who can assist you with the pros and cons of your unique situation and help you decide where is the best place to have your business incorporated.