This is a two-fer:
1) Randomize the spawner received from purchasing the spawner kit
2) Have a "Hostile Spawner Kit" and a "Passive Spawner Kit" for Economy

Considerations:

• Not all mob types should be represented in the kit, similar to Skyblock where not all mob types are options when changing a spawner
• Ratio of mob types (per Kit type) to prevent flooding of "rare" mobs. Since Pig is the default (and I admit I have not purchased the kit because I'm relatively certain it's all Pigs), Pig could still be the highest ratio by a large margin. Example Breakdowns below:
  • Passive/Neutral Mobs
    • Axolotl - 0.05%
    • Chicken - 8.00%
    • Cow - 10.45%
    • Fox - 0.80%
    • Glow Squid - 0.05%
    • Horse - 2.00%
    • Mooshroom - 0.10%
    • Panda - 0.05%
    • Parrot - 2.00%
    • Pig - 74.00%
    • Rabbit - 0.50%
    • Sheep - 1.00%
    • Squid - 0.50%
    • Turtle - 0.50%
  • Neutral/Hostile Mobs
    • Creeper - 40.00%
    • Drowned - 0.05%
    • Enderman - 12.50%
    • Magma Cube - 12.39%
    • Shulker - 0.01%
    • Skeleton - 15.00%
    • Slime - 2.00%
    • Spider - 10.00%
    • Wither Skeleton - 8.00%
    • Zombie - 8.00%
    • Zombified Piglin - 0.05%

Included things that spawn naturally or that are already in the crates at a high %, otherwise there's no way to balance the "rares".

Another alternative would be a Spawner Kit that is just rares, but you only get one a month instead of one a week. This kit could be either random like above, or it could be a specific spawner each month for everyone. It could even be a 2-year cycle with 24 spawners to keep it from cycling out of control (although I can't come up with 24 spawners that wouldn't include a large number of "standard" mobs that kind of defeats the purpose of this type of kit). I didn't come up with any kind of weight, but a 1-year cycle of "rare" spawners at 1/month could be:

• Axolotl
• Blaze
• Drowned
• Frog
• Glow Squid
• Guardian
• Mooshroom
• Panda
• Shulker
• Tropical Fish
• Turtle
• Zombified Piglin

 

Sounds great, I'd definitely like to feel as if I'm getting my monies worth when paying out £50 on a kit that's worth around 100k in game. But for hostile spawners, we'd actually need them to exist first

 

Cool idea, but hostile spawners aren't available yet and it might take a little bit to decide the prices on them.