Just to be clear, that effect buff on Surprise Defense would only happens once, if you play the character using Suprise Defense and he remains alive at the end of the turn, he comes back to your deck. If he dies, instead of going to graveyard he becomes exaust for 3 turns, remaining on the battlefield, but if he dies again he goes to graveyard normally. The exaust for three effect only prevents death one time.
Aboutut how those changes would break the game, right know the game is "balanced" because 70% of the cards are completly useless trash (yes, the 70% percetage is a pun with Calamities Posts, but most of the cards are indeed pretty bad and useless). With these changes the game would be balanced not because almost anything is weak and underpowered, but because most of things are usefull and strong, this would make decisions about what to put and what to remove from deck making more significant.
Right now each faction has only 1 or 2 really cost effective cards per card cost (ie: 1 really good 5 cost card that pretty makes every other 5 cost card dispensable), making everything usefull in some way would force the players to giveaway some card with strong power to focus on other powerfull card which have the same cost but have more sinergy with the rest of the deck.
If everything is OP nothing is OP. If everything is UP, 2 or 3 cards on each faction sustain the burden of the entire faction on their backs.
The majority of useless trash card can became great and playable cards simply reducing their 1 resource on their cost or giving a +1/+1 buff.
Maybe some of the changes that I am suggesting was too much, but right know the game has lots of stronger almost cheater cards wihch have a colossal cost efficience and the games is still playable, not optimal playable but with some balance.
These changes will mostly correct problems with some specially underpowered factions that lack high cost eficcient cards and only relies on a few OP cards (i.e. DOD), at the same time other factions are extremely meta dominant because they have some of the best cost efficient cards on the game (i.e. Flame Dawn and G.I., almost every meta deck I see use one of those two factions as a core).