I imagine Q-ships can be muddled in with escorts and become pointless once all merchants are assumed to be armed anyways.
As for Auxiliary cruisers, sure would be nice, but the model at the moment seems unsuited to model them.
From a submarine point of view, yes, I suppose Q-ships would be a waste of space once unrestricted sub’ warfare is initiated and, it seems, weren’t that much use in real life either, but against aux’ cruisers, maybe not so much.
So much depends upon what the actual model for merchant shipping is & how it works. If, as I believe, merchant ships are actually going to be on the map now, then an aux’ cruiser would be able to engage individual, unescorted merchants. This would force a convoy system for important routes in much the same way as large numbers of individual sub’s spread all over the place did. But the less important, less
frequent or lower bulk routes may not all be able to be convoyed-up & escorted in this way.
Once sub’s are operating in packs and stalking large, escorted convoys, the Q-ships become the natural adversaries for the aux’ cruisers. Even carrying some cargo.
How to implement them? (aux’ cruisers & Q-ships)
Give them a very low visibility. They would be difficult to “detect” (read – identify as an enemy) even with radar. You have to get close to spot them. Fairly high convoy attack and about the same sea attack values as a cheap light cruiser but, the important difference, is that they
can stay at sea for many months.