I believe emperor thaurissan is the main reason that the deck is as explosive as it is. It's a card that inherently allows mega combos to occur, pair that with a reliable card draw engine and you get a consistently explosive and powerful combo deck. I don't believe that nerfing the draw engine would be a good idea, if the desire is to not destroy the deck. If the deck cannot cycle it's cards then deck falls apart because the individual strength of each card is very poor, but when combined they become extremely powerful. Any nerf to card draw would result in a complete collapse of the deck.

If you had the ability to stop emperor thaurissan from activating then it would be much harder for patron to win. I think making his activation occur at the start of the turn punishes noninteractive decks like patron and freeze Mage that do not fight for the board. Since they will usually be behind on board by the time they need to play emperor, it will be difficult for them to ever achieve a single activation. This would raise the skill cap of the decks even higher and make them able to burst for far less. A deck like handlock would be much different because they may already have a giant and a drake on the board by the time they can play thaurissan. Druid might have shredder and druid of the claw. I realize this may make emperor borderline unplayable, but the card should not exist in the first place, if blizzard's design goal is to promote "interactive" gameplay. This card promotes the exact opposite.

In a world without emperor: On a full board (opponent has 7 minions) at ten mana you play warsong, double frothing, whirlwind that's 7*2 and four more from the base stats totaling 18 damage, comparable to a slightly scaled up druid combo while requiring more pieces. Frothing and three whirlwind effects would deal 23 damage, close to the double druid combo while requiring 5 cards. If deaths bite is equipped it becomes more damage but that's not a "from nothing" combo, and could be stopped with taunt (they can't play execute too) and weapon removal.

This change also makes bad matchups far worse,and keeps the good matchups in place. For example, to beat handlock you often will need to 0 mana execute, whirlwind several times and play Frothings, this becomes impossible. In the face hunter matchup nothing changes because emperor is usually too slow.

While I personally don't believe that patron is as popular on ladder as many people argue it is, it is still the strongest deck in the game. If a nerf does occur it should leave intact the ideas of charging patrons, making a board of patrons, allowing for some burst from frothing, and in rare circumstances the mega empty board OTK.

Tl;dr: make thaurissan activate at the start of the turn, punishes uninteractive decks that do not fight for the board, and can be useful in decks that will develop the board. This prevents the super OTK combos from an empty board and makes it much harder to combo and execute through a taunt strengthening the effectiveness of handlock and ramp druid. The matchup against hunters probably remains the same.