There is a reasonable level of hotel capacity around EMA but I do know that at least one other organsiation is considering investing in further hotels space in the area because they see the need.
The race track has not got the F1 Grand Prix (and that was a debacle) but that would have been relevant for one long weekend a year and you would not build a hotel for that alone. However, the track operates virtually 365 days of the year (probably about 360) and there is also the exhibition centre, both of which are on the up since the return of the track to the Wheatcroft family ownership. Room capacity for some of the bigger race meetings is certainly in short supply.
Castle Donington isn't just the airport and the track, though, and there are current plans in place for further expansion of the business and light industrial areas in or near the village, which may attract a need for further hotel rooms.
Many of the hotels in the area are of the budget variety, maybe there's a need for at least one slightly more upmarket hotel with conference and leisure facilities (and good sound proofing!)