I would expect to see Ryanair and MAG building a very strong relationship. Despite the history, Manchester is now Ryanair's second largest UK airport with the third being East Midlands. The top three Ryanair bases in the UK this summer are all now MAG airports. Bournemouth is Ryanair's smallest UK base, but Ryanair account for a much greater percentage of it's passenger traffic, even more than at Stansted!
I would be surprised to see Ryanair pulling back at Stansted. BAA are already charging the limits they are allowed so I doubt we will see any significant rises in landing fees and other airline operating costs. I think for now, Ryanair will be very open to further expansion as in their eyes, the BAA ownership couldn't get much worse. I would hope that because Stansted already has very good infrastructure, a large airy terminal, lots of natural light, lots of space, a good layout, good range of hotels, car parking etc. that they recofnise it's an airport for LCC's and can afford to lower landing fees to a degree as less needs to be spent on infrastrutural improvements at Stansted.
For a time, I can see them using Manchester's profits to help Stansted get on the right track, but doubt it will last. In terms of what Stansted could, or certainly should attract are the non-based carriers at Luton (Blue Air, El Al, both of which used Stansted in the past), I would be surprised if Wizz moved as Luton is a lower cost airport, smaller and less direct Ryanair competition, the dream airport for Wizz I would imagine.
In terms of Gatwick's operators, FlyBe is the big opportunity who are currently unhappy with Gatwick's pricing structure. Stansted currently lacks domestic services and I'm sure MAG would want FlyBe at Stansted to operate some of the key domestic destinations like BHD, IOM, JER etc. Southend may also start to feel a bit of pressure if MAG can attracy EasyJet to grow again but I think that will be a much longer process.
All in all, I think the future now looks a lot brighter for Stansted.