Runway
The extra runway investment would presumably allow:
More payload availability for marginal destinations. These would presumably include Egypt and the future charter destination - Dubai.
It would allow ETOPS aircraft to reach the US East Coast and Canada. Luton has had almost no transatlantic charters over the years. 3000m would make these more likely. By cutting out stops at Bangor, Florida flights would be more cost-effective.
A longer runway with high-speed run-off taxiways would increase the movement capacity. I don't think there's room at present for full-length taxiways to both 26 and 08. If there were, capacity would increase.
In short, little change to the European routes, but the chance for more medium and long haul.