If you press tab to autocomplete commands where there are multiple options, the sorting order seems to follow no clear rules.
Reproduce:

• Type / into chat
• Press tab, see autocomplete suggestion
• Press tab multiple times and look at the suggestions

Result: /defaultgamemode, /msg, /summon, /spreadplayers, /achievement
These are neither sorted by number of usages (defaultgamemode is pretty rarely used, summon, tp and execute are way more common), nor by alphabet (either /? or /achievement would be first), nor by order of adding into the game (first were server commands like /op in alpha).
This is not a suggestion, but a valid bug whenever MC-1797 is a valid bug, because it's very related.