Although the future of Newquay Airport is still uncertain I would have thought there is potential for more routes from Cornwall.
At present there are the WOW daily flights to LGW and FR to STN, plus Scilly Skybus to the Scilly Isles. From next spring there will be additional WOW rotations from NQY to DUB, BRS and LBA.
NCL and EDI might be considered as destinations from NQY if WOW wishes to expand operations from that airport.
PLH is in close proximity to EXT that has started a range of Flybe routes all over the UK in the last 12 months and this, together with skyrabbit's point about PLH's runway, might limit options from there. Also Air Wales has a decent range of flights from PLH to ORK, DUB, ABZ, NCL and LPL, some via CWL.
I doubt that WOW would want to start too many flights from PLH (or NQY for that matter) each morning to destinations via BRS because how many people use the PLH-BRS sector at present? "Not many" I believe is the answer.
The alternative would be to base an a/c at BRS, but would they want to start what would be another base? If they did I can only think of NWI and ORK, and possibly LPL, as destinations either not covered at all or not adequately covered from BRS at present. That is unless they take a leaf out of Flybe's book at EXT and SOU and go for something like Brest and/or Cherbourg, but I presume that would be summer only.
Interesting dilemma for WOW at PLH though. They have a potentially decent catchment at PLH but a very limiting runway, plus an established competitor there and a bigger airport forty miles away. The alternatives are an even bigger airport and catchment a hundred miles or so away, or an airport with a question mark hanging over its head with a smallish catchment forty miles in the other direction.