To answer this we have to look at the past heritage of these vessels in the post-WWII environment.  LSD's evolved from their WWII designs to carry more and larger landing craft, with the Harper's Ferry/Whidbey Island-classes capable of LCAC operations.  LPDs were developed primarily to support helo ops.  LSDs generally could land more cargo and heavy equipment, while LPDs transport more Marines. 

 

Looking forward I could easily see then next generation of LSD (2020s timeframe) as an evelution of the LPD-17 design, adapted by reducing aviation support for cargo and perhaps an expanded well-deck.  It also depends on the future ARG make up, if the LHA(R) has no well-deck, and how many ships are included.  I do not think it likely, but you could see two future mini-ARG types/split; one with a LHD & LPD and the other with a LHA(R) and LSD-X (or whatever designation), for somewhat balanced aviation and landingcraft capabilities.

 

Just some random thoughts...

The LPD is the main platform for the new AAAV. i think a good ARG will be a LHD, LHAR and 2 LPD.

the navy has a minimum  number of LCACs  6 or 7 needed to support an ESG. with the new LHA-R being developed the make up will be interesting If they use the new LHA then they will have to use  a LSD  Whiblkey Island class that can carry 4 LCACs and the LPD can carry only  2. the LSD  Harpers Ferry class can only accomoidate 2 LCACs. So they will have to be paired with the LHD

 

LHA-R no LCAC

LSD 41 4 LCAC

LPD 2 LCAC

 

LHD 3 LCAC

LSD 49 2 LCAC

LPD 2  LCAC