There are a number of small improvements we could make to the resume token format, although these may require a bump in the version number. For instance:

• Never explicitly encode any of the redundant info beyond tokenType for HWM tokens.
• Encode tnxOpIndex as explicitly null for non-txn events. This will generally ensure that txn events from the same shard are contiguous in the stream; currently, the first entry in a txn will interleave with any events from other shards at the same clusterTime.
• Consider hashing the eventIdentifier as SHA256 or similar. This would limit the resume token to a fixed size, but we would lose some potentially valuable diagnostic information in the process.