After the Stansted sale, perhaps a shift of some of those Gatwick routes to Stansted since EasyJet are dropping the key ones like Alicante and Barcelona? Only Ryanair remains on those now, not even a Thomson charter or anything like that!
The only problem is that Monarch, amongst many other airlines like this view it as the tradition to serve Gatwick since that was designated the holiday/leisure/charter flight airport of London and a move to Stansted would be ultimately unrealistic. It's an approach that airlines need to drop and airlines should cater for the under-served markets, rather than overcrowding airports with too much direct competition having duplicate routes from several airlines. For example there will now be about 15 daily departures from Gatwick with 5 airlines to Barcelona in the summer, Stansted and Luton will have only up to 2 departures with 1 airline each! There is no way that demand for this route is more than 5 times stronger round Sussex than in Essex.
Particularly after the Stansted sale, Monarch could acheive much higher yield at Stansted than Gatwick, but it's this biased sort of "tradition" that makes them soldier on at Gatwick, where they have now just become obscured as one of a load of operators, they don't scream out as a highly attractive option with the likes of EasyJet and BA alongside them. They would at Stansted since many would happily pay that bit more to avoid flying with Ryanair!