I agree with the Burning change suggestion, but I think the Bonus damage should be disabled for that case, It would be too powerful in my eyes.
And I think a trigger like:
only cards with the same or less Rarity as the Played card can be Discarded.
would be useful, to prevent destroying enemies Rare cards Regularly.
On Undead, I prefer Progressors idea, but why not making the condition a little versatile, by saying:
one not Undead card is Replaced by Abyssal scavanger.
so it can hit the Undead Deck too,
With Brigend I agree too, its a little weak, I join the idea of (1/3/5), or at least raising the Brigand token gain a little.
Also Nature, Restoration and Destruction cards could have a little effect, like Legend:
Nature: there is a 25% Chance to raise random Recource by two,
Restoration: there is a 25% Chance to raise stock by one,
Destruction: there is a 25% Chance to reduce enemy Stock by one
or something like that,
another Idea for Nature/Restoration/Destruction:
there is a 20% Chance that next Card is a Nature/Restoration/Destruction of higher Rarity.