I was thinking that if Qatar operated the A321LR in the future could Luton sustain a flight to Doha connecting to key markets like Pakistan, Dubai, Australia and many others.
Qatar Airways could easily operate their new fleet of 787-8's from the airport, I however doubt this will happen anytime soon...
The airline has only recently started a 7th daily rotation out of Heathrow, these are operated with a mix of A380's, 777-300's, A330 and 787. To increase capacity for the moment, they can easily replace the A330's and 787's with larger aircraft.
Also Birmingham to Doha by Qatar Airways goes online in a few months, with a daily 787 service, so opening up Luton would encroach on this market as well as Heathrow.
Also before they secured so many slots at Heathrow, they always used Gatwick as an overflow for peak time travel periods, to add capacity. So if they were to expand at another London airport, I would LGW would be the preferred option.