Getting to SDs and attack subs seems like it creates 2 classes of units with overlapping roles... invisible units with 30 strength that can only be countered by the same unit type.
Destroyers and I think airships can detect subs, along with other subs. Only SD's can detect SD's.
Earlier subs had destroyers and other subs to worry about, now it seems subs only worry about other subs, and SDs about other SDs...
Subs will always have to worry about standard Destroyers. As of 3.17 they are now a dead end unit that never upgrades or goes obsolete.
Does the SD still intercept aircraft? can you build the old destroyer after teching to the new? does this leave the carrier with fighters as the only source of naval air defense?
In 3.17 I believe SD's were stripped of their ability to shootdown planes and Destroyers were therefore allowed to be built until the end of the game, they no longer upgrade to SD's. SD's are a weird unit, but great for scouting or (I imagine) blockading. They don't seem to really have much to do with Destroyers anymore actually.
A 2 movement artillery would be nice, so it wouldn't slow down the stack to bring along, but I wouldn't go out of my way to tech to them
Mobile artillery are pretty sweet. Once the enemy develops AA defenses, you want to use your precious air force more carefully and having arty around to bombard defenses to 0% without any risk of retaliation is very useful again. Obviously, as you indicated, there's a trade off in that you're delaying other techs.