ToME: the Tales of Maj'Eyal

i feel a good way to make the game much more replayable would be to include more "set of items" bonuses, with at least 3 objects that already exist in game.

We could start with unique items : some of them dont have to have an apparent combination and this "secret set" shouldnt be obvious until it happens, the bonus should be sweet but no overpowered.

Another idea would be to improve low lvl unique items when they combine with other high lvl ones... that way they would still have some use in late game.

Such as Summertide/Wintertide and Summertide/Wintertide Phial?

yep but the problem is both go in the same slot....

I think you misread what I wrote.
One is a weapon, one is a light source.