A Dragon/Steel type is the antithesis of what the objective of CAP 5 is; taking two of the strongest types in the metagame and mashing them together does not encourage use of a more underused type.
I would like to echo Reachzero's comment regarding how CAP 5 fares vs. Dragons:
Originally Posted by Fat reachzero
If we give it the wrong typing, it will be guaranteed to be unable to switch in, and it will be extremely difficult to find a way to actually kill those Dragons. Specifically, I would say that Dragon itself is a terrible typing to consider, totally destroying the whole point.
I would also like to echo his sentiment that a Dark
typing has potential, dealing with Lati@s very well and eliminating a large threat for Sun teams (which seems to be the direction that this CAP is leaning toward). However, I would also like to mention that CAP 5 should not be "Effective Dragon Counter", but more of "Opens up unorthodox options while not being weak
to Dragons/rain/etc". Dark falls a little short in this respect because it indirectly encourages Fighting as a counter, and if we decide to circumvent this by means of a Dark/Ghost typing, we're basically just making a better Spiritomb - and here I struggle to see how a Dark/Ghost CAP 5 will strengthen just the underused parts of the metagame while not being used on standard teams as a mon with no weaknesses that destroys Lati@s.