DeviousQuail
Hero
The Cavalier in my current party is really good. Between Unwavering Mark, Warding Maneuver, and Hold the Line he excels at everything you'd want a tank to excel at. Hold the Line is almost broken when fighting certain monsters. We're playing Rise of Tiamat and his ability to stop a dragon from running reduces/takes away one of the most dangerous aspects of fighting a dragon. Other classes and subclasses have "tanking" abilities but the Cavalier's combination of ease and effectiveness is, imo, second to none.The Cavalier in the Xanathar's Guide to Everything has some interesting options to play the Tank/Defender Role, though I've never seen them played at the table. Has anyone played a Cavalier character? How did those mechanics work during gameplay at the table?
The funny part for our group is that we also have a Paladin with the Protection FS and a Rune Knight with the cloud rune. I almost regret choosing War Magic over Divination for my wizard but then I remember my DM is a good person who doesn't deserve that much pain in his life.
To the OP, I miss the Swordmage as well, but I think 5e's current defender abilities are varied and effective. I wouldn't look to add more just for the sake of adding more. But if they did bring in a swordmage or warden I'd expect nothing less than some new defender abilities.