I'm missing some important parts that I would argue any (binary)format needs.
- a magic header
- a version number
- a crc or data corruption check
Additionally I would argue one would be better off writing a custom text parser instead of parsing a binary format in this case, this is similar to the debate about unixlike config vs windows regedit. I would prefer something other then json but sill readable as txt.
Even the json example provided at the bottom can be minified from 418 characters to 166 by replacing the field names with single characters and removing the spaces. Almost all of the savings in this format come from not including the the field names and having a position dependent layout. You can choose to have delimiters or arrange for a byte to indicate the type(+size) for example the following string encodes the example data almost (85bytes vs 80bytes) as efficient but is still readable and supports utf-8 interpretation.
123456789;LeroyJenkins;60;alliance;p,100,200,300;i,999,1,1;i,45,100,0;a,s,120;a,a,45;
You may optimize it further by allowing recurring entries and allowing assumed values from a defined default and only sending delta's can be dependent on the type of data you are expecting.
{ "itemId": 999, "quantity": 1, "isSoulbound": false }
i,999,1,0
could become:
default = { "itemId": 0, "quantity": 1, "isSoulbound": false }
{ "itemId": 999}
i,999