I was wondering how people would feel if standard changed to force 40 cards to be the new deck limit?
Hearthstone meta gets stale real fast and a big part of it is because the deck limit is too small to have a lot of variation. A pretty common evaluation of new cards is "its good but I wouldn't spend a deck slot on it". At 30 cards, decks are barely big enough to fit all the cards they need for their archetype so there is very little variation in each version of a deck and you are pretty much guaranteed to draw at least one of every card in your deck so there is not that much variation what happens in a game either.
Would adding more variation by forcing decks to have more cards be a win for the game or is the relatively low amount of variance part of Hearthstone's charm?
The size of expansions is linked to that base 30 card deck. If you want 40 cards all the time they'd have to make larger expansions to provide enough cards. Just look at Renathal decks having to run crappy cards quite a bit to hit the 40 right now.
Changing a fundamental rule in Hearthstone permanently is a terrible idea on its own. Not only that, most decks thrive off of only having 30 cards in their deck, by forcing everyone to change to 40 will significantly decrease the power of most decks while making already strong 40 card decks even stronger in the meta. If your idea of variation is hoping your opponent gets unlucky with inconsistent draws for the archetype they're running, they'll end up getting frustrated and quit playing the deck. Then they realize the pool of decks who do well with 40 cards is quite small and constricts play style heavily. Then they get bored and quit the game.
Renathal is great because he allows certain decks to thrive with a 40 card, 40 health limit, but he isn't required to run, which allows for players to enjoy their other decks.
A pretty common evaluation of new cards is "its good but I wouldn't spend a deck slot on it".
That's also a part of the reason Renathal was made. If you can utilize the extra space well, good for you. But forcing decks that don't need extra space to put weaker cards than their starting 30 is bad.
More cards won't lead to more diversity, only more randomness and more expensive decks. People would just run the same 40 cards instead of the same 30, in most cases. As a consequence it would also heavily nerf slow decks that more often rely on drawing specific cards.
There are other ways to approach diversity. For example overloading archetypes with more cards than the archetype can use. That way you need to choose what to include and exclude, and the optimal choices can change over time if many cards are situationally good.
Another one is relying less on archetype synergies. I think a really good example of a current healthy archetype is Ping Mage. You can play it with just Dawngrasp, Brann and Apprentice for a late game win-con, or you can add around 5 other cards for the full package. Either way there is tons of room to either lean further into pinging or combine it with other archetypes. An example of an unhealthy archetype would be Mech Mage, which has like 26 mandatory cards and almost no wiggle room, which not only means the deck won't change now, but also will remain the same in the future unless we get more Mechs.