Placing an extra 319 or 320 would make economic sense. Regardless of where it comes from, its going to make money from the word go. Thousands of people are looking to rebook for S10, and my bet is they would rather pay easyJet the money.
I'm sure it would too, but given EZY haven't expanded at GLA for years, I wonder if there is still a continuing problem with charges, or they just want to sit tight and wait to see if BAA end up being made to sell the airport, presuming they'll get a better deal from new owners.
Nevertheless, if they can get any kind of fee deal at all it would seem a no brainer to come in and replace some of the GSM routes. TFS in particular operated virtually daily on a 738, was I believe consistently over 90% full and, going by whenever I looked, not cheap either!
FR have also I think left some city routes at PIK recently to focus on EDI, but they operated for years from PIK and I'm sure would work from GLA - FCO, PSA, maybe DUS, etc.