ToME: the Tales of Maj'Eyal

Helm of the Dwarven Emporers
Pick of Dwarven Emporers

These two items are not a set. They must be a set.
You could also add the Cuirass of the Thronesman though there's downsides and that's kind of awkward.

Agreed! The cuirass is awkward in general, maybe it could loose that healing mod penalty in a complete set?

We've also got Wintertide (tier5 sword) and Wintertide Phial and Summertide (tier5 shield) and Summertide Phial. I'd love to see those made into sets, or maybe make a big 4-piece set (would require one of the phials become a misc instead of a light).

There are a few obvious should-be-sets out there that could use coding up.