Well I think the diversity in high level would increase if only we had a ****CASUAL MODE!
The thing is, people can’t do proper testing unless they have a consistent way to test with friends, which is far from being the case for the majority of the player base, even in s-rank/diamond. So if people can’t test they just play what has proven to be the strongest deck in the meta, no testing>no innovation>no diversity. If only people had a way to do serious testing they would try to explore other possibilities.
The other factor is that people are so busy grinding the ladder every single season that they don’t have time to do proper testing. I personally think that a ladder reset every month is WAY too much , people who don’t have much time to play the game but have the level to reach S-rank can basically only afford to ladder all the time. They don’t have the time to do testing: so no testing>no innovation>no diversity.
Also monthly reset cause other problems like the ladder being completely messed up and almost unplayable for beginners at the start of every season. Beside people would be more inclined to climb the ladder if they knew that they are going to keep their ranking longer.
What I suggest is to increase the duration of the season by one month, people would still be able to reach S-rank without having to ladder as often so they can enjoy and test on the Casual mode that should have been implemented a LONG TIME AGO. So people will have more possibilities and time for testing: more testing>more innovations>more diversity.