For it to be legit BRT, it would be a pretty significant investment. You would need a protected bus lane. That lane would need to be legitimately protected, meaning concrete barriers between it and lanes. It would need to be in the inner drive right lane since the southern portion has housing. There would need to be stations, not little bus stops; those stations would require metro-like fare gates (unless the city decides to go proof of purchase, which I can't imagine happening) so that passengers can enter and exit freely from all doors. To get the stations set up without further restricting traffic flow, and to handle passenger volumes, you'd probably need elevated stations with stairs and elevators on each corner of each stop.

You would need to reconfigure every intersection so that the BRT lane either bypasses the intersection above or below, or so that the buses automatically trigger a barrier, like how trains trigger barriers. You'd further need to create barriers to prevent other vehicles from the possibility of entering the lane.

All of that is probably doable...on the boulevard. Where you run into trouble is at Broad. If you terminate the line at the BSL, you fail to create a one seat ride to center city, which would eliminate a ton of ridership (you can already take a two seat ride as is: virtually every cross street on the boulevard in the neast runs a bus to FTC, so this would only be slightly faster, if at all).

If you want to take it into center city, how do you do it? Running down Broad is one option, but you don't have the space to do the same kind of things you can do on the boulevard, plus you have engineering concerns with the BSL underneath. You might buy some advantage with signal priority, but traffic will slow it down. Can't do lane protection unless you use the innermost lane, do away with parking, and ban loading and unloading, but even then that only works if you express all the way to center city, bypassing Temple.

Aside from Broad, there's no road that even makes it feasible unless you run the bus down the Roosevelt Expressway and the Schuylkill, but that, again, bypasses Temple.

Yes, a boulevard subway is hard, and no, it doesn't solve issues north of Cottman, where everything was built to support car based living, but it could have a to transformative effect to join North Philly with the greater Northeast. A one seat ride, protected from traffic, from the city line all the way to Temple and City Hall? That's massive.

Then, to make it more meaningful, you go further, and it doesn't have to be crazy! Create two shuttles between the boulevard subway and the FTC: one at Oxford Circle, and one at Bustleton Ave/Levick St. The shuttle would be free.

Last, create an express bus on Cottman, with stops at Bustleton, Castor, Algon, Five Points, and ending at Ryers. Keep the same rolling stock on the Fox Chase line, but convert the fare structure to transit vs RR. Now you have linked the entire lower northeast to center city, north Philly, and itself in a way that has never been possible. The far northeast gets the short end, but there really isn't the kind of density you need to support additional rail transit West of the boulevard by the time you hit Pennypack.