I don't know much about Raleigh to be honest, but would a downtown arena when PNC deal runs not better for the Canes attendance? Getting a new owner with a vision to for something like this would go a long way to making sure the team stays in NC.
A hockey-sized arena would make more sense downtown than a soccer stadium IMO.
I think Nashville makes a good case study. For those who haven't been, take a look at downtown Nashville on Google Earth.
Hockey arena -- right on the main tourist strip, practically shoulder to shoulder with major hubs like the convention center and state capital.
Football stadium -- across the river in a sea of parking lots, accessible by footbridge but otherwise in its own district.
Indoor arenas are multi-purpose venues which can host all sorts of events, and are physically smaller in scale so that they don't require as massive of a footprint. They usually draw much smaller crowds so that you don't overwhelm the city's infrastructure. You can stick one of them in a city district without being highly disruptive, as in Nashville and Charlotte, and know that it's going to be a hub of activity year-round and most nights of the week.
By contrast, the outdoor stadium is very limited in use, much larger in footprint, and usually will be intended draw massive crowds. I would assume that an MLS team would be aiming for the 20,000+ range with an upside closer to 30,000 for big events. That means major changes in downtown infrastructure, if it's to be done well.
If I ever manage to get over fear of flying I'd love to come to Raleigh to see the Canes (I'd have been there already if it wasn't for flying),
FWIW, an 8 hour overnight flight from the UK means you can essentially knock yourself out with medications and sleep through the whole experience.