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rock was a good type in gen 1 when normal-type moves made up the majority of most movepools.
the two biggest issues holding it back are an overall increase in movepool diversity, specifically in regards to coverage, and getting overshadowed by other defensive types. two of steel's three weaknesses are shared by rock, and fire is not that common an offensive type. gamefreak is probably never going to change the type chart again, but if they did here's how i'd buff rock:
>remove its fighting or ground weakness. which one? not sure, you could make a case for either of them. maybe both.
>give it the dark resist that steel lost in gen 6. dark is a very common coverage type, especially on early-game normal types
the aim of these changes is to slot rock back into its role as an early game wall that resists all the "basic" attacking types, while cutting down on shared weaknesses so there's a potential reason to use it over steel.