First I will go over all 6 classes starting with the classes that received an update. Secondly over the seasonal theme. And last whatever is on my mind.
1. Wizard Class
Typhon 6p change is heavily welcomed. Typhon is the Wizards speed set so I would like to see any kind of movementspeed increase to define it’s role even more.
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Hydras deal 4,000% increased damage and gains 5% attack and movement speed for each Hydra head alive.
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Fragment of Destiny: Your Signature Spells attack 50% faster and deal 5 times more damage. You gain a Spectral buff whenever you land an attack with a Signature Spell. Hydras deal 30% increased damage for each Spectral stack. Max 10 stacks. [20-30] [30-45]%
Hydra is in an almost good spot just a little more damage and speed would feel realy good.
In general the Wizcommunity wishes for a viable channeling and primary build. And I selffishly would make jumps, if all Arcane Orb runes got buffed up to be on par with the FO rune. Especially Spark and Scorch. 
2. Monk class
Pattern of Justice how about making it even faster with incorporating the Tailwind rune into the set? Of course advocating for a tiny bit more damage is never wrong.
Only thing I miss is that Sweeping Wind adds nothing to it’s damage.
3. Barbarian
The increase in damage of the 6 pc bonus of Horde of the Ninety Savages is well done, but why not either increase the damage of
- the 2pc from double to triple damage
or - increase the damage of an allready existing supporting legendary by some xxx% damage, too.
or - a small 25 % elite and another 15% Boss damage multiplier on the set.
It feels like an elite hunting set, so that’s a little missing in my experience.
4. Demonhunter
Not much to say, except I would like to see RoV triggering Spiketraps.
Besides that I only can fell the urge to see GoD6 6pc be buffed by 1-3 tiers worth of damage.
The seasonal Theme
It’s great and should be permanent with changing powers to choose from. It’s like the perfect tool to bring back old seasonal themes just a little more spiced up. Getting the RoRG or 4th cube slot unlocked via the altar of rites would bring excitement for years to come. Maybe that’s even the plan allready.
What else came up to my mind?
I would really wish for, that new Seasons focus more on two and a half of
- Adding new stuff to the Basegame f.e. reworked/expanded Hellfire/Uberbosses or updated crafting system
- More focus on outdated skills, classes, items and sets. To make it short: a bigger focus on balancing.
– Quality of Life improvements. F.e. what Raxx, Maxroll and the whole Diablo community proposed together.
Season 27 introduced some great fun and not totally broken powers. Would love to see a lot of them coming back on an item or incorporated in a set. F.e. more Magics missile for the Wizard, the amazing FotH power for Crusader, your amazing pet gang for the Witch Doctor, etc. Did I mention Witch Doctors? They should really become a focus point for the next season.
Links to some proposals:
Wizard buffs
Even more Wizard suggestions
And even more
A lot of suggestions above made it allready into the game or are outdated by now but still worth the read.
I have not found concentrated wishes/proposals for other classes but feel free to drop your suggestions or links in here.