Not sure domestically what's left observed, I think most routes are covered?
Internationally certainly from BHX there are quite a few left un-served: Berlin, Stuttgart, Milan, Lyon, Knock, Toulouse and Nantes plus summer only to Bordeaux, Avignon and Biarritz.
Doubt some of these will ever return but surprised no one has looked at Berlin and Milan.
I think the new Flybe, if it indeed ever takes off, will have a tough job in terms of network. Just 2 or 3 key bases might work, one of their many mistakes was so many costly bases and the amount of HOTAC, taxis, hire cars and positioning flights needed to crew the shortfall of crew that seemed to affect every base. Having so many bases, they were trying to be jack of all trades but master of none.