I'd say you can add the required changes into the Forge PR - As long as the required memory usage isn't that bad. Based on the size, that's roughly 65k times the currently possible array size (and from 1 byte per entry to 4 bytes). Should definitely be respected when finding a solution for that

Going above the $2^{31} - 1$ limit of integers would cause far more issues in several places. Therefore, I assume we don't need to consider longs.

For Paper, using int arrays would be mostly wasted memory, so I'd rather have multiple implementations of the relevant parts.
Exposing ints rather than chars in return types or parameters is likely fine, but internal storage should be specialized depending on the amount of block states.
That means we could probably in a first step update the current interfaces (so the usage char is limited to a small scope, like everything based on CharBlocks but nothing else). Then, implementing IntBlocks (and subclasses) should be pretty much straightforward.

Do you have any information what's a typical amount of block states in e.g. mod packs like All The Mods? That would be interesting for context.

According to the output of Block.BLOCK_STATE_REGISTRY.size() there are 625113 blockstates in All the mods 7. When I ran it with Conquest-Reforged, create, and mr.crayfish's furniture mod, I was around 424000 blockstates. Conquest-Reforged adds a lot.

This old PR can be closed, as a new PR has been created with a clean implementation fixing the issue and is created off the newest codebase.

Correction, I thought I was closing the PR, not the issue. Woops!