New rare custom "artifact" items would be added to the game as drops. These would be extremely rare items/tiles with unique powers or abilities, they would be an "ARTIFACT" rarity and be coloured dark red with sparkly particles coming from its UI box. These would be available in limited quantities (ie, as a daily quest reward) or can be obtained through very specific limited-time means. Upon viewing the item in your inventory/bad, the item's info would contain the item description and also a number specifying how many obtained copies of this exact artifact are in existence and. This artifact could be generated by an algorithm or could be custom made by the dev. Artifacts cannot be sold. The power of an artifact, if not a tile, would be considered as research, and therefore would be disabled if a base's "disabled research" is active. Some artifact Examples:

Unlimited Scaler Source Tile
Modifier that Increases all Laser' Tower's Laser Width by 50%
Damage Artifact that Increases Global Damage by 8%
Cool-Looking artifact that is just for bragging rights >:3
Boss-Less Portal
(Personally, I think each Artifact should have a cool artifact-y sounding name lol)

I think that this style of items would add more interest to the game, especially for late-game players who receive 10 legendary items for breakfast.
These items would not be suspect to "inflation" (like how the value of things decreases to a player as they progress in-game, as they receive more of them). They would add a suspect of excitement for events as a challenge could be presented to complete for a cool new item. They could even be used as rewards for "secret codes."

Well, that's the idea. Hope you like it.