Either a route is viable or it isnt. There is no magic and markets cannot be 'rustled up' from nowhere.
Not so - Ryanair especially play on being able to get one over on their competitors. A good deal at both ends makes a route much more viable. Then there is the question of a/c usage - if there's a few hours to fill, but not enough time to nip out and back to the costas, send it somewhere cheap and local - but make sure you cross water, otherwise the cheapskates will take the coach.
There is always VFR, there is always some need for a business meeting, and there are always curious tourists looking for new places. These can all help fill a few rows in their own way, and people who use one airport to continue by land to another city can help make up the balance.
I content that there is already a 'market' for most European major cities from BHX - and certainly for our tired list of capitals not served (BER, MAD, STO, ATH etc).
With the right aircraft, frequency, deal at each end and marketing, all of the above, and many more
could work.
Conversely, hundreds of routes each year which 'look' popular get dropped, usually because the market 'exists', but the yields are poor.