What other Eastern European/Baltic destinations are MAN missing that could be a winner?
Some that come to mind are Tallinn; Vilnius; Katowice; Poznan; and Cluj as well as Bucharest. Some of those have been tried in the past and the routes pulled, but it doesn't mean they couldn't work in future.
The difficulty is that they operate from competitor airports already by various carriers, so it needs an airline to take up the challenge. While easyjet fly to cities such as Berlin and Madrid from both LPL and MAN, I don't see Wizz doing the same with its destinations. Routes served from DSA and BHX but not LPL would be less of an issue for them I guess.
Edit: Of the possible routes mentioned, it seems Wizz only fly to KTW from LPL. Ryanair fly to Vilnius and Poznan from LPL but they do duplicate one or two routes I believe. Blue Air have flights to Bucharest and Cluj.