For those who are unaware, the F/A-18D trades some of the fuel capacity of the C variant for a second seat.
Although the US Marines used it in combat roles most operators used the D variant as a trainer.
Ever since the A-10C, DCS has lacked a trainer worthwhile flying in combat and capable of teaching deployment of modern weapons. The F/A-18D is the perfect aircraft to fill this void.
Instead of having the trainee share their screen to the instructor, the instructor can check and change the aircraft and weapon configuration, and demonstrate how to perform maneuvers all whilst sitting in the same aircraft as the trainee. This would be particularly useful when teaching skills such carrier landings, smart weapons employment and ACM. The skills learnt in the F/A-18D would be directly transferable to the F/A-18C and act as a great basis of knowledge when transitioning to other modern multi-role fighters. Better yet, the F/A-18D would be perfectly capable in a modern DCS mission (even without the WSO equipment found in the USMC planes).
I don't imagine it would take much work for ED to make since most of the work would be done once the F/A-18C is out of early access. As for selling it, I think selling it as a standalone aircraft and in a bundle with the F/A-18C would work well if ED charged extra for the bundle (proportional to how much extra work was required). For owners of one of the standalone aircraft a discount could be applied for the one they don't own allowing current F/A-18C owners to become instructors and graduated F/A-18D pilots to transition to the single seat fighter.