I have many decks, and not all of them are useful in all modes. But I like to play long/hidden with random decks from my list.. and playing this with something like Frenzy rush deck is pain and suffering. So I got an idea: what if you could mark a deck for different game modes? It should function like existing replay filters, but only when you create a random game.
For example: a player has Mage deck for long/hidden games, Far sight deck for long/non-hidden ones, Frenzy deck for any normal one and Illusion deck he built only recently and hasn't tested it yet. So filters for each deck would be like that (long-friendly-hidden):
Mage: Include, Ignore, Include
Far sight: Include, Ignore, Exclude
Frenzy: Exclude, Ignore, Ignore
Illusion: Ignore, Include, Ignore
So when he creates a long friendly hidden game with random deck, he can create a game with his Mage or Illusion deck.
Of course, there should be filters for AI and challenges as well. Also, I don't think it's too hard to implement, since we already have replay filters that work similarly.