I get the letter grading is boring, but I think it's so common because it works, and it works flexibly.
- F
- E
- D
- C
- B
- A
- S
- SS
- SSS
That's 9 different "strength" rankings you can work with, which gives you a wide spectrum of what your monsters are gonna be. Even if you use a different naming system - like the metal rankings or numerical rankings, it's more or less just a change in packaging. As long as there's no meaningful difference between "D grade" and "silver grade", you might as well use the one people are more familiar with (unless metallurgy is specifically part of the culture of your world).
If it were me and I specifically wanted to make a different kind of ranking system, I wouldn't start with the monsters. Monsters do not rank themselves. Monsters just exist. Rankings are arbitrary, artificial classifications created by humans (et al) for their own convenience. So you should probably start by asking:
who within the story is the one that created this ranking system and
for what purpose.
Traditionally, the ranking grades tend to apply to adventurer guilds, so they're specifically meant to be quantifying the threat level of the monsters in order for, often illiterate, mercenary characters to be able to quickly and easily understand whether or not this is a fight they can fuck with. A D-level adventurer understands they can fight D, E, F monsters, and maybe C in a party, but above that they're better off leaving it to others. The rating system works in tandem with the adventurer rankings. Another facet is that numerical rankings are also, you know, used in grading in education, so it evokes presumed audience knowledge and familiarity with someone's capability. A straight A's student is a very capable student, so it's easier to immediately feel, as a reader, that an A rank adventurer is very capable, or an A rank monster is very powerful.
So who is the ranking for? Who made it? For what purpose?
Maybe the system was originally actually just a classification system made by a researcher, and it was adapted and simplified over time for mainstream usage in monster hunting for strength ranking (or whatever you're gonna apply it to)? So you can have categories of monster types (land monster, flying monster, aquatic monster etc.) and then subgroups within those classifications, like Nahrenne suggested. For myself, I'd do something like "Goblin: L4S -- Land, 4th rank, Social", which informs "they're a land-based monster, they're inherently weak, but they tend to congregate into social groups, aka can overwhelm with numbers even if their individual strength is weak". This is the sort of ranking I can both imagine having been an anthropological classification made by a researcher for shorthand notes that ended up filtering down into hunting classifications. This is just an example of thinking about the source and working out from there, rather than trying to come up with a ranking and then working backwards on how to apply it to the story/world.
Whatever you do, if you're posting on SH, I would suggest making use of the glossary feature the further you move away from known systems so readers have something to refer back to, and put some footnotes in. They'll learn it over time if you bake it into the world properly but they'll need constant reminders to start.