This could make for an interesting plugin. Since NodeBB already stores recently logged IPs, there may be some similar code to examine (or at least to provide ideas from). Unfortunately in my forum's scope, there is the occasional proclivity for users to change their names on a whim - whether they just want to make a shift, or in some unfortunate cases, a user is scammed and someone tries to distance themselves from the incident with a new alias. Given NodeBB's user-friendly URLs, it seems like it would be easy for someone in the second situation to change their name, and avatar to "hide" in a large pool of members. While I find this concerning, it's to be expected given my users.
Getting back on a happier note, I think an awesome feature would be to list previous usernames on a user's profile, with some ACP customization (how many to show, enable or disable, etc). This idea may not be fore everyone though, so it is best suited as a plugin, I believe.
Along the same line of thinking, it would be really neat if when a user changes their name, the system would update all the posts they were tagged in. Sounds cool, but this could be pretty resource intensive, so it's not exactly the most feasible; still neat. This idea comes from reading several old discussions on other forums where users would change their names months later, and based on the quoted posts in old topics, it would for a moment, look like someone popped in, had a conversation and disappeared - or getting rid of confusion regarding who @MrWaffle is in a topic posted by @Schamper.