and likewise, the Greater Undead Vaults are ridiculously onerous to clear.
Harder than level 50!

This would be a good change.

It makes sense...

Agreed, GGV is a piece of cake for any char, way out of GUV league.

From the thread about new greater vaults, there is a giant vault suggested composed like this:

This would probably be suitably difficult.

Or I suppose you could just sort of upgrade what is already existing:
Ogre lord -> Ogre king or ogre emperor
Hill giant/Stone giant -> Fire giant/Frost Giant
Fire giant -> Frost giant jarl
Frost giant -> Titan

Reference monster list -
Giants:
0 - Ogre Lord
1 - Hill Giant
2 - Ettin
3 - Stone Giant
4 - Fomorian Giant
5, A - Fire Giant
6 - Frost Giant

Undead:
0 - Ghost
1 - Spectre
2 - Vampire
3 - Ghost Lord
4 - Master Lich
5, A - Lich King
6 - Emperor Lich

Red Dragons:
0 - Fire Lizard
1 - Fire Drake
2 - Red Baby Dragon
3 - Fire Elemental
4 - Red Dragon
5, A - Ancient Red Dragon
6 - Great Red Wyrm

Suggested monster list for updated Greater Giant Vault:

0 - Hill Giant or Ettin or Fomorian Giant
1 - Stone Giant
2 - Ogre Lord
3 - Stone Giant Lord
4 - Frost Giant Jarl
5, A - Fire Giant King
6 - Titan

Titans (not greater) as ultimate guardians for greater giant vaults are proposed in several different RFEs. This RFE is even more relevant now, when we have 2 guaranteed greater vaults in the game. In my last game they both were giant vaults. Kinda boring this way:(

+1 on this with addition of Ogre Emperors being a possible 4/5

Changed.