DUS is slot restricted, yes (as is FRA). Lufthansa has built up a nice network there over the past three or so years and most Star Carrier serve the airport. There are, however, very limited long-haul connections - only EWR, MIA, ORD, YYZ and PEK. There are, however, rumours about additional long-haul destinations to be added. It really depends what potential Lufthansa sees at places like GLA - apparently not enough people originating from GLA made long-haul connections at LHR to make the route viable. But generally speaking, DUS seems to be a very unlikely choice (despite the fact that it is the only Lufthansa hub with a NCL route). I guess they will try all they can to accomodate flights at FRA and only if they are unable to, look at alternatives.
However, as far as FRA is concerned, don't be fooled by runway #4. It does not mean a 25% increase in departures as traffic flow apparently restricts the number of additional movements.
OTOH, there are also ZRH and - to a lesser extent - VIE as potential hubs. For Lufthansa, Zurich is as much their hometurf these days as Munich or Düsseldorf.