That would probably mean the most likely candidate for a new system (should it get to that point) would be Motorola, considering that you have Bucks, MontCo, Philadelphia, and the State of Delaware with Moto TRS and Delaware County (inlcuding the separate PSAPs in Haverford Twp and Media) with it's Moto conventional system and West Chester PD with it's Moto P-25 conventional system. (I can't speak to Berks County, but it might not matter as outside of the City of Reading, they're still VHF low and high band.)
Please, don't suggest that ChesCo look into OpenSky. That would be a monumental disaster for scanning in this area (not that it's that great to begin with). It doesn't matter that Lancaster County is about to make it's "deal with the devil" in a matter of months (even though they were supposed to go online at least a year or two ago).
Speaking of which, ChesCo's Comm 1 mobile command unit does have an OpenSky mobile to communicate with the state and (once they go online) Lancaster County and West Chester University (State System of Higher Education). I would imagine that the 9-1-1 Center also has an OpenSky mobile as well, so it wouldn't be neccesary for the county to get an OpenSky system.