Secret Cave looks interesting—obvious comparisons are Mill (cantrip DfB) and the next-turn effect of Swamp Hag, i.e. +$3. Where Mill is good at hitting $5 early, Secret Cave looks good at spiking to $6 or sometimes $7. It probably plays well in a Minion stack that's either lightly trashed down, or trashed back up with green, or that just doesn't do enough with an even Minion split; there it probably plays about the same as Mill.
Obvious Nocturne synergy is Faithful Hound. I was waiting for doggie's other shoe to drop. In general, DfB and other action-phase payload plays well with draw-to-x, such as Cursed Village.
Den of Sin is a non-terminal Enchantress without the attack. Cards (and actions) are at their most precious at the start of your turn; I probably want it whenever I'm building an engine, 4 is good and 6 is better, but I probably don't want it to be my only draw.
Tragic Hero seems the... hardest to use for great effect. In BM it's a Smithy for $5, which is slower than a Smithy for $4. In an engine... meh, you'll want action phase payload because it'll play somewhat like a draw-to-x because you don't want more than x; gaining a treasure anti-(self-)synergizes with that kind of deck.
I'm notsureif Guardian passes the Lighthouse test or vice versa; I guess with good deck tracking, a tactically timed Guardian can help you hit $n+1.
Sacred Grove is a bigger and better Woodcutter. When that's your +buy you take it.
I have no idea about Tormentor.
Leprechaun: A Gold gainer for $3—eh, if only I knew how bad Hexes are on average. Opening double Gold gainer seems pretty okay if you're playing a money game.