I would consider looking at the AMEX Platinum (unbranded - unassociated with Delta)
You would get access to Skyclubs (useful when connecting through DL Hubs at Atlanta and Detroit), you would get priority pass which entitles you to use
3 different lounges at IAD - The Air France/KLM Lounge, British Airways Galleries Lounge and the Turkish Airlines Lounge (That has been getting rave reviews). You ironically would also earn more RDM than the delta co-branded cards since the Platinum card earns 5x on airfare.
A lot of people still hold on to the Delta Platinum for the MQMs and the companion cert, so I would consider keeping both the AMEX Platinum and DL Platinum.