I really like this idea, but I'm wondering why the rarity of an ordinal is only based on being the first of an event.
There might be other ways of looking at how rare an ordinal is. For example, the ordinal that corresponds to the first one millionth sat being mined. That's pretty cool and rare, but things like that are not part of this theory.
Is there a reason why?, I think there might be different ways of considering how rare or unique a specific ordinal is.
Anyone can decide what sats they consider to be exotic. You could decide you really want a sat mined during block 500,000. Or maybe you really want a sat that was stolen in an exchange hack. I think the "official" rarity tiers are ones that are convenient numbers (the same way that people who collect dollar bills for their serial numbers might want ones with a certain number of zeros).
By the way, there are a few blocks that I want sats from, so I set up a bounty: https://www.rot13maxi.com/ordinal_bounties/first.txt
There certainly are different ways of determining rarity, and I definitely encourage people to come up with their own.
The "official" rarity levels, which are first in block, and first after difficulty adjustment, halving, and conjunction (difficulty adjustment and halving happen on the same bloc), are just suggestions, and I wanted to keep them as simple and objective as possible, so they're all based on things that come from the Bitcoin protocol themselves.