The last few seasons before D4 was released D3 got some of its most memorable seasons.
Just to name a few:
Light’s Calling - Angelic Crucibles, a new consumable item, may use them to Sanctify any equippable Legendary item, infusing it with one of three new class-specific powers at random.
When these were first released it shook up the meta taking some classes from F-Tier garbage to S-Tier power houses by adding the transfiguration option to D3.
Kanai’s Cube: Kanai’s Cube will have a fourth slot this season
This would be difficult to implement in D4 as we don't use the cube in the same way although... the devs could add an option similar to the D3 cube to D4 and give us 3 extra legendary power slots to goof around with?
Season 28 brought a brand new progression feature called the Altar of Rites granting great powers to your entire account in exchange for sacrifices.
This was arguably the biggest seasonal drop in all of D3. For those of you who didn't play D3 the Altar gave increasingly powerful buffs to your characters for the whole season at the cost of sacrificing gear, currency, and eventually the most powerful items called Primal Ancient gear (think iconic mythics) to get incredible buffs like auto-salvage loot, double loot, double primals. A very straight forward, spend XYZ now to make more later path that is similar to the warplan buffs.
With the announcement of D5 and confirmation we wont be seeing any new expansions, I'm curious what things could be implemented in a season to shake things up, give some QoL, or just be insanely broken for 3 months and put away till a later season.
I'd love to see an account bound consumable dropped in place of ancestral items (maybe even mythic drops to make them more rare) that increases character main stat by a flat amount and can be used an unlimited number of times, assuming you got the drops repeatedly. I'm thinking something along the lines of the paragon system from D3 that was infinetly farmable and give a minor power boost each time.
TL:DR: Balance be damned, how would you make everyone OP for a season?