If you are looking for something fresh in the Gwent card game available in The Witcher 3: Wild Hunt, the Gwent Plus Plus modification is perfect for you.
This mod adds multiple new cards (i.a. siege engines, a Doppler, and a Nekker warrior), but most importantly, it changes the already existing cards and adds new abilities. For example - Villentretenmerth after being killed returns to the battlefield as Borkh Three Jackdaws. The entire card game becomes more balanced and all the decks are comparable in terms of their strength - in the original game, the decks differed from one another significantly.
The creator of this modification suggests to start a new game after installing this mod, but he also proposes two other solutions - buying new cards from Yoana in Crow's Perch or writing the updategpp command in the game's console.
This modification can be installed either with Nexus Mod Manager or manually - by pasting the contents of the downloaded archive into the Mods folder.