This would probably flop, and therefore kill the business, but it would have to be Southend to London (and Heathrow) - again.
Starting at Southend Airport, it would serve the town centre and then major points along the A13 London Road up to the A130, then divert into Pitsea and Basildon, and then run fast along the A13 to Lakeside Shopping Centre before crossing the M25. From there it would run fast to Victoria, calling only at Lodge Avenue for Barking, Canning Town for the Docklands, Aldgate, Embankment, Victoria, Earl's Court, Hammersmith and Heathrow Central. I might even throw in a non-stop extension to Reading, partly to compete with the RailAir but also to restore a link that existed ~30 years ago.
The Southend to London coach route traditionally fared quite well, until c2c suddenly became very reliable; now, after a period of uncertainty, the 090 NX route - a fairly new addition to the NX operation- is the only one. What this operation does that others have not done quite so well, if at all, is provide South Essex with links to Southend Airport, and also provide faster journeys between locations within the M25 and Southend.
Another one I've thought about would be a link between Southampton and Bristol Airports, via the A36. It would intermediately serve Totton, Salisbury, Warminster, Beckington (for Frome and Trowbridge, with through fares available onto First's 234 and 267 bus routes), Bath City Centre and Brislington P&R (again with through fares, this time to Temple Meads and Bristol City Centre); at the Southampton end it would terminate at the City Centre before/after the Airport.