1. If OTK is going to be a thing, then there should be more cards that interact with your opponent's hand, deck, and graveyard. Currently OTK decks are not interactive because their win condition does not require any board presence. There are lots of creative ways this could be done without completely making the archetype unplayable, such as cards that stuff your opponent's hand or deck with useless cards, force draws, discards, steal or transform cards in these zones, secrets, etc
2. Improve pace of play. Roping one turn to make a particularly tough decision is okay... roping every turn is not. Maybe a have a chess clock like mechanic, or a shorter base time for each turn with any extra seconds that are unused carried over from one turn to the next.
3. I would like to see a larger base set. As sets are rotated out of standard, select some cards from those sets to add to the classic set. Key tech cards like Kezan Mystic would be good candidates and would make it such that there wouldn't need to be a slightly different replacement version reprinted every other year. Other "toolbox" cards that add a new dimension to the game and are well balanced (like the rogue secrets) too.
4. How about an occasional buff for some unplayed cards that add a neat mechanic to the game but are unplayable, cards like kidnapper? Especially where it would shake up the meta and enable new deck archetypes.