Ascension has come out with quite a number of expansions, and most of these have introduced new mechanics. Dreamscape is 9th in the series (released 2015), and some of the surrounding releases suggest that Stone Blade Entertainment was scraping the bottom of the barrel, idea-wise. Sadly, this means that many people did not buy Dreamscape specifically because it had a new, messy-looking mechanic. As it turns out, this is one of the best sets.
Dreamscape keeps the mechanics of the original game and is compatible with all other expansions. Additionally, it has a new type of 'currency' and another card type that interacts with the usual game cards.
At the start of the game each player is dealt 5 dream 'dream cards', of which they select 3 for their dream deck and discard the rest. Throughout the game the center-row cards will provide opportunities to gain 'insight' tokens which will allow them to buy cards from their dream deck, or add new, random cards to their dream deck. The dream deck cards, once purchased, will either enter play the same as any card purchased from the center row, or will perform a one-time action that benefits the player.
Which, admittedly, sounds unnecessarily complicated. But it works, and works quite well. A player can ignore this new mechanic and still win the game... or double down on it, or use it to whatever extent they like. And whichever path you chose, it's fun.
While Dreamscape can be played on its own, it also combines well with the other sets, and I find it works particularly well with the base game and/or with Ascension X: War of Shadows.