bit64::integer64 can be awkward to work with in R (one example: https://github.com/apache/arrow/issues/7385). Often in Arrow we get int64 types from compute methods or other translation methods that auto-promote to the largest integer type, but they would fit fine in a 32-bit integer, which is R's native type.
When calling Array__as_vector on an int64, we could first call the minmax function on the array, and if the extrema are within the range of a 32-bit int, return a regular R integer vector. This would add a little bit of ambiguity as to what R type you'll get from an Arrow type, but I wonder if the benefits are worth it since you can't do much with an integer64 in R. (We could also make this optional, similar to ARROW-7657, so you could specify a "strict" mode if you are in a use case where roundtrip fidelity is more important than R usability.)
Likewise, uint32 and uint64 could be kept as integers and prevent the conversion to double that is currently implemented.