Im not sure if bladders are the solution. Ideally I think at sea replenishment solves the problem as it doesn't take up any space on the LHD.To significantly increase the Jp5 capacity you would need ~1000m2 of space. Replenishment could be monthly, fortnightly or weekly depending on operations.
Problems are things like Aegir 18r wouldn't be able to replentish the LHD jp-5 more than once, completely. So if we got the 18r and we are trying to run fixed wing ops and are operating two LHD's, at a high intensity then we have a problem.This can be avoided by making sure we have replenishment that can support more than ~500,000 gal of Jp-5 (and 1,000,000 might be more appropriate).
SASWannabe posted a great post back in 2011 about fuel capacities of carriers including replenishment.
http://www.defencetalk.com/forums/navy-maritime/hypothetical-carrier-buy-ran-10410-58/
I would also think about ship magazine and compatibility with F-35 ordinances and capacity.
It again depends on how we want to use it. Most likely we will be deploying 2 x LHD together to get the capability we want, providing a combined fuel reserve than a Wasp LHD. With 2 ships finding place for say 2 or 3 F-35's on each ship would provide significant capability (and I would imagine at least 1 would be on deck ready to go on each ship).
As a pure aircraft carrier. I would imagine 2 x LHD (surge), each with up to 12 -F35B's + NH90's + UAV would provide very significant capability to anywhere in the region or globally. To sustain that commitment one of the LHD could be relieved at a time, refreshing maintenance stores, engines, air frames, weapon magazines, personnel etc. With two ships, you could run as the USN does and have a day carrier, and a night carrier. When operations slow, you then go back to a single ship deployment which you can sustain as long as needed without interfering with maintenance or training.
IMO it is better to be able to replace a ship after a month or two, with a fresh crew, fresh, supplies, fresh stores, fresh facilities, than try to maintain a long deployment of your single carrier. Given that we are most likely going to operate in the region (pacific or Indian oceans) that approach shouldn't be as much of a problem as travel time will be short and relatively low risk.
But I guess why I think 3x LHD make more sense than 2x LHD + 1 carrier, is that you don't compromise your amphibious capability at all to do it (in fact you enhance it as you can now train that amphib capability). The only thing that would require a big reconsider is say PoW or an America class purchase (or gift) as they are in a different class of ship. IMO highly unlikely, and its unlikely that even if it got something like that we could operate it effectively (crewing, replenishment, training, maintenance etc) unless we were on a war footing. Where as a 3rd LHD is just another one off the rack and fits in with all our long term planning, logistics and training. In fact if we were serious and we didn't want a local build, we could put in an offer for Juan Carlos herself which Spain could replace with another build.