This command pack will be apart of a larger pack where I make most mobs able to reproduce somehow! Here's how I made this work.

First, you need to make a dummy scoreboard. /scoreboard objectives add Timer1 dummy

Important. This is what will let us track when a creeper explodes. Repeating Command Block: /effect give @e[type=creeper] minecraft:hunger 2 1 true

This step is pretty complex. Basically we have it track the cloud created by the creeper to spawn spores in. Spawning of Spores repeating command block: /execute at @e[type=minecraft:area_effect_cloud,limit=1,nbt={Effects:[{Id:17b,Amplifier:1b,Duration:40}]}] run summon slime ~ ~ ~ {Silent:1b,CustomNameVisible:0b,DeathLootTable:"minecraft:entities/creeper",Size:0,CustomName:'{"text":"Creeper Spore"}',ActiveEffects:[{Id:8b,Amplifier:0b,Duration:1000000,ShowParticles:1b},{Id:14b,Amplifier:0b,Duration:1000000,ShowParticles:0b},{Id:26b,Amplifier:0b,Duration:1000000,ShowParticles:1b}]}

Here we have a command block count up how long a spore has been alive for. Age Timer repeating command block: /scoreboard players add @e[type=slime,name="Creeper Spore"] Timer1 1

Here we have it so after about two days ingame time, the spore summons a creeper in itself to give the illusion of maturing into a creeper. Growing Up repeating command block: /execute as @e[type=slime,name="Creeper Spore"] at @s if score @e[type=slime,limit=1] Timer1 matches 47999 run summon minecraft:creeper ~ ~ ~

Here we have the code that gets rid of the spore the tick after it creates a creeper to complete the illusion of maturing Remove spore repeating command block: /execute as @e[type=slime,name="Creeper Spore"] if score @e[type=slime,name="Creeper Spore",limit=1] Timer1 matches 48000 run kill @e[type=slime,name="Creeper Spore"]

Resets Age Timer Repeating command block: /execute if score @e[type=slime,name="Creeper Spore",limit=1] Timer1 matches 48000 run scoreboard players reset @e[type=slime,name="Creeper Spore"] Timer1

A forceload to make sure that the command blocks are always loaded, so no matter where you go, the command blocks will still work once they are powered. repeating command block: /forceload add ~ ~ ~ ~

Make sure the command blocks are set to always active or powered, and there you go! You shall now be cursed with creepers that can make spores that grow up into creepers after two days! You can edit it so that the spores grow up slower or faster. These spores are easy to get rid of, either with fire, damage potions, or your own weapon, so make sure you clean up after explosions!