Stand availability seems tight overnight. During the day there are throughs but I think more based aircraft at the moment would be hard unless carriers agree to do more bussing on a daily basis. Already there are issues with widebody stands, causing things like the Cathay being towed of to allow singapore on, or forcing widebodies onto remote because of lack of contact stands.
There are definitly times when there is capacity, but also times when there doesn't seem to be. The good bit is most of the time there seems to be runway capacity (although at times there might be capacity issues in or out, but rarely both at once) but then if you've got no stands available that doesn't help. Also eventually ground capacity will probably be the limiting factor.
Now all of the above is completely my own speculation and could be completely wrong.
Norwegian moving in would be an interesting move. However to fight what battle? Go head to head against Thomas Cook on long haul to the US? Fight ryanair, easyjet, thomson, monarch and jet2 to spain and the canaries? I'm sure there are places left to fill in the market, but I also fear a blood bath if too many carriers move in once the inevitable dip in the economy comes again.