To me, definitions of Album and EP are already fuzzy.
Some bands will call a 6 tracks 35 minutes duration an EP, some others an album, or a mini album. Sometimes bands uses EP and mini-album interchangeably, for the same release.
I’m always bothered with those, for example, sometimes there’s a release group for an EP (let’s 4 tracks, 25 minutes), but the band releases an extended version of it adding 3 tracks and 15 minutes (7 tracks, 40 minutes), since that’s an extended version of a previously released EP, it goes in the same RG. But would an user search in EP marked RG for this release?
I’m not against adding mini-album as a new RG type, but I’m not sure it will help anyone over EP / mini-album.
Slightly off topic, but current layout is awful on artist’s page, listing by RG type then by date (with singles before EPs), and adding a new type will not help in this regard.
MA is using similar types (note, no mini-album either) but list releases in chronological order first, bold for full-length, I find it much easier to read.
Back to topic, I have no strong opinion about adding or not mini-album, but I don’t see much bands using this over EP, perhaps it depends on music field though.
For sure, choosing between single, EP and album is the most annoying part of adding a release (group) for me, sometimes I would intuitively add something as EP but notice the band calls it an album, sometimes that’s the reverse, sometimes I have no indication at all (and then rely on number of tracks / durations, without having a clear definition)… Overall that’s not that bad, but I don’t feel RG type is a very valuable information, since a short album and a long EP don’t differ much anyway.