A reworked Slime Rain would have 3 tiers.
Slime Rain Tier 1
The first tier is like the slime rain we already know and love, with two differences:
Spiked Slimes can spawn alongside the other slimes, making them no longer Expert-exclusive. However, King Slime himself can still only summon them on Expert and above.
Gold Slimes, previously exclusive to celebrationmk10 and everything seeds, can now rarely spawn during this event, being slightly rarer than Pinkies.
Slime Rain Tier 2
You have killed the King and now the slimes are angry! The second tier is unlocked while still in pre-hardmode, requiring the following criteria to be met:
King Slime has been defeated atleast once, either by completing the first tier of the Slime Rain or manually summoning him.
At least one of every type of pre-hardmode Slime has been killed, except Shimmer Slimes, Windy Balloons, and seasonal Slimes.
On the second tier of the Slime Rain, almost all variants of pre-hardmode Slimes can spawn, including variants from the first tier and biome-exclusive ones. Some of them, however, with changes: Dungeon Slimes won't have a Golden Key inside them, and Lava Slimes will not leave lava behind upon death (regardless if the world is on Expert mode). Aside from that, new enemies will also spawn:
Slime Pawns - They wear a kettle hat and yield a shield. They can use that shield to completely block attacks, similar to Granite Golems and Biome Mimics.
Slime Knights - They wear a knight helmet and yield a jousting lance. They can dash towards the player and deal damage proportional to the distance they are from the player. The farther away they dash, higher the damage gets.
The second tier of the Slime Rain does not have a boss. Instead, the criterion to complete it is killing a certain amount of the afore-mentioned enemies. Additionally, Slime Rain-exclusive enemies are not affected by the Royal Gel accessory, and will always attack the player on sight.
Slime Rain Tier 3
The slimes are calling for reinforcements! The third tier is unlocked on early hardmode, and requires the following criteria to be met:
The Wall of Flesh has been defeated, meaning that the world must be on hardmode.
The second tier of the Slime Rain has been defeated atleast once.
Atleast one of every type of hardmode slime has been killed, except for Hopping Jacks.
The third tier of the Slime Rain is the last and most difficult one to complete. In addition to enemies from previous tiers, hardmode slimes and two new enemies also spawn, alongside an already-known fellow.
Slime Bishops - They wear a bishop hat, yield a staff and can fly. They can shoot magical beams towards the player, afflict the player with random debuffs, and heal/buff nearby enemies (regardless if these enemies are slimes or not).
Slime Rook - A mini boss, it is a moving tower driven by Slimes that shoots canon balls and flaming arrows, and spawns random types of hardmode slimes.
Queen Slime - In addition to being summonable with a Gelatin Crystal in the Hallow, the Queen Slime can now also spawn as the final boss of the Slime Rain, requiring a certain number of Slime enemies to be defeated.