I think that the route which could work best is just simply having National Express or Airport Bus Express extend their Stratford - Stansted service into Cambridge. This would make a service which is more similar to Greenline 757. While not 'Oxford Tube' every 10 minutes, I think that by having the mix of passenger journeys, that being Cambridge to Stansted, Stansted to Stratford and Cambridge to Stratford, this may be viable. Terminating in Stratford, Stratford is a destination in it's own right but you do also have tube connections into Central London, I think that running into Central London direct could ruin the reliability and make the journey time too long to be competitive.
There are now services in both directions every day. From 29th April, it will be 3 trips per day from Cambridge to London (5 on Thursdays) and returning London - Cambridge it will be 3 trips on Mon, 5 Tues, 4 Wed, 3 Thurs, 2 Fri, 3 Sat, 3 Sun.
Does the airport do concessions for coaches or a standard departure fee? I only ask because Luton does concessions and this stops most coach operators from serving the airport, they have to bid and provide a minimum level of service and stuff so for Luton, the awards only went to Arriva (for Greenline) and NatEx. If Megabus or Flixbus wanted to serve Luton, they wouldn't be able to get access. If Stansted is the same, perhaps Flix/Mega can't serve the Airport because of the concession/agreements in place.