There's a Reason why Domestic Clubs Failed @ BWI
The new pay-per-use lounge at BWI is in the space formerly occupied by UA's RCC (UC) which was the last domestic lounge to close at BWI.
The BWI market is low-fare based and caters to a different demographic than DCA & IAD. Unless DL intends to go after a different demographic. it's highly unlikely that it will build out a club, especially when there is one available for just $17.50.