Madrid is a classic example, and in my view it's still now underserved with the 4 x weekly FR flights (5 x weekly for a month this summer).
I do wonder if MAN has rather neglected flybe for flights to some European destinations. The airline is seemingly focused on BHX, increasing capacity to MXP, HAJ and STR, and promising new routes from there. Flybe withdrawing from FRA and BRU from MAN in recent years was perhaps understandable, and while they offer several daily services to PAR and DUS, I do question whether the single flights to HAJ and MXP are likely to make the most of potential business traffic to those cities (I appreciate they have competition from FR to Bergamo).
STR, as has been mentioned, is now an obvious void on the MAN departure boards and cities such as Lyon with just a weekly flight from EZY are hardly over served. These destinations are also perhaps more suitable for, say, a 80 seat a/c rather than a/c with 130, 150 or 190 seats to fill.