Some of the old Seer items.
Problem is I don't know that they'd be the most useful or most sought after.\
If they were to put some of the Artifacts and Replicas on a rotating cycle, I think you would see more life in Doom and Ilshenar Champ Spawns.
Not to phase out current Replicas and Artifacts, mind you, but to have them not spawn for, say, 6 months to a year, then spawn again for a month or two, then not spawn for another 6 months to a year.
Replica versions of other old EM or old Seer items could exist along-side new items. To throw all of the items into the same mix would, I feel, be a mistake.
There is, of course, one gigantic problem with both of our ideas, and that's that there's so many items and properties in the mix currently, and Imbued items on top of that, that how many more items could the game possibly support?
But, still, it's been shown time and time again that it's difficult to make UO players do things without rewards (it was like this even pre-AoS to some degree), and I, for one, have a hard time thinking of rewards other than new items of one type or another.
-Galen's player