Monarch could base an additional one or two aircraft at LTN and just serve destinations they already serve from their other bases, such as Dubrovnik, Venice and Madiera.
If Wizz were to base aircraft and crew at LTN, they would probably have to pay the crew more than Eastern-Europe-based personnel, so there may not be a benefit to them. As it is, they are constantly adding to their 'first wave' arrivals (now 13 on most days), which takes advantage of the time difference between the UK and their destinations.
Thomas Cook are extremely unlikely to base aircraft anywhere new, as they are struggling financially, and reducing their fleet.
I agree that Air Berlin are unlikely - they have dramatically reduced UK operations, moved from STN to LGW, and joined OneWorld.
I suspect that the airport finance director won't want to attract too many 'struggling' airlines. However, there may be potential for European-based airlines to reduce their London costs by transferring one or two flights a day away from LHR, much as El Al has done. There would be some duplication, but I could see that being an attractive option for the likes of Alitalia, Aegean, Turkish, Air Portugal. Their only problem would be direct competition from EZY, which they don't suffer at LHR.
Of course there could be a wild card - a Middle East carrier looking to offer connections to the Indian sub-continent perhaps, or an airline using Gatwick such as Norwegian wanting to split their London flights and tap north and south markets separately (unlikely until the economy improves).
I quite like the idea of FlyBe transferring all or some of their London operations from LGW - I'm sure competition from EZY would be no more challenging at Luton than at Gatwick.
Ryanair are looking at taking a 25% stake in Stansted, so one wonders what effect that would have on their choice of London terminal - if it happens. They have operated the four based airframes for 7 years now with big changes in routes flown, but essentially no increase in the size of their operation. They probably want to keep a foothold at LTN to maintain a presence in the market, and keep STN management on their toes, although I can't see major growth from them unless EZY leave.
Finally, Thomson. They have slowly reduced seats from LTN from the heady days when they (as Britannia) had 10 or more aircraft based. Their market is in long-term decline, so the best we can expect is continuity.