I think Labyrinth is usually going to be really bad, and then occasionally be brokenly strong. Also, I think never-ending games might come up too often. I expect the main use for it is in over-drawing engines. With cards like Secret Chamber, Vault, Cellar, Storeroom, Warehouse, probably others, you can draw everything, discard a bunch of Labyrinths, re-draw them and repeat to generate a ton of VP. With a deck of 2 Tacticians and 8 Labyrinths (and possibly the trasher that got you to that point), you can generate 8 VP every turn. Fortunately in that case the game might still end since if both players go for it they'll split the Labyrinths and have some room in their deck for other cards. But that's the sort of situation I'm worried about.
But I hope you can make it work out because it's a pretty cool idea.