Just educated guesses but seeing how much New Fantasyland expansion helped increase capacity at the Magic Kingdom - I could only imagine that capacity would increase by a decent amount.
DCA is a bit different I think. For the sake of example (*these aren't actual numbers, just simple ones), let's say DL's capacity is 10,000 and it averages 9,000 guests. DCA is built next door, adding a capacity of 5,000 so the two theme parks have a total combined capacity of 15,000.
But you can't be in DL and DCA at the same time, so even if you park hop, the majority of that added capacity will be impacted in DCA and not DL, since that is the 'new' offering. You might have 14,500 guests on property now, but only 9,500 are in DL and the additional 5,000 in DCA.
Now say a new land with a popular IP is added to DL with a capacity of 1,000. DL's capacity goes from 10,000 to 11,000... but now DL's crowds increase from 9,500 to 11,000, because DL has the 'new' offering. This creates a larger and more direct crowd growth than an indirect overflow of DCA.
The park will be more crowded for sure, but as someone on the east coast used to WDW line lengths...crowds won't bother me