That idea is very interesting and in terms of creating your own decks it can increase the fun very well.
But when i look at Xi Ascended, i already see a problematic card in 1 purity OoS or DoD. On the long run, balancing those cards out and allowing some sort of creativity but narrowing the space for toxic playstyles/decks is a hard task.
Sometimes less is more - currently the game would maybe be way more balanced with less dominant options provided by 1 purity cards and more dedication to 3x purity decks!
For now, thats one of the things i like in Gwent right now the most. Full dedication to a commander and a faction, but still some different archtypes of playstyles and deckpossibility left. Sure, hard to compare each games with eachother... but just wanted to add that