Acting for Two: In the secret video of extend, Asami Shimoda is both Rin and Len.
Bad Export for You: A minor example, as it is limited to the demo version, but the Project Mirai Deluxe demo could only be played once in Europe, while the American version can be played 30 times. Later subverted when SEGA admitted that this wasn't intentional and that players who want to play the demo more than once should contact the eShop customer service.
Colbert Bump: General rule of thumb is that if a song with relatively little popularity makes it into a Project DIVA game, expect it to experience a popularity spike.
Early-Bird Cameo: In Project Diva on PSP, Hello Planet has a mini-game version based on the non-extend music video. This mini-game can unlock a second, alternative ending for the music video it's based on. As of the end of 2015, it has not appeared in any game with an alternate PV.
Edited for Syndication: Very often, songs are pared down in length to keep them within the 3 to 4 minute mark so levels don't drag on too much. This even applies to the theme songs. Strangely inverted for "Poppippo!", which was too short and had to have its length doubled by adding borderline nonsensical English lyrics.
mirai does not do this to any of its songs, even songs that were also in DIVA. This has the unfortunate side effect of creating Marathon Levels with unnaturally long tracks like Reverse Rainbow. Songs that wind up being ported back to the main series via Arcade usually undergo this process, like "Piano Girl".
X has Special Live Mode, which allows you to combine three songs onto a single setlist and play them like you would a Medley. This trope takes effect to keep the total length below limit as opposed to making you play three full songs in a row.
2013 saw the release of Megpoid the Music#, a similar rhythm game for the PSP featuring the popular Vocaloid GUMI.
Gumi also appears in selected songs in Project mirai, however she has not been licensed for Project Diva itself.
And in 2014, IA/VT Colorful does the same for IA on the Vita, courtesy of Marvelous AQL.
Miracle Girls Festival is essentially Project DIVA, but as a Massive Multiplayer Crossover with several anime franchises. Especially notable in that Miracle Girls is also made by SEGA, runs on the exact same engine as DIVA, and features very similar gameplay.
Finally ended with Miku Flick and Project DIVA F, both versions, although the PSP and first 3DS game remain Japan-only. DIVA Arcade is available outside of Asia in select arcades (specifically, some of Round 1's U.S. branches).
Just a day before Project Diva f 2nd was released in Japan, SEGA announced a physical release of the game in both North America and Europe. Unlike Project Diva f, Both the Japanese AND English versions are being released in parallel to each other, rather than staggered.
Also, the DLC is being released, semi-randomly, near constantly for Project Diva F 2nd in America and Europe.
In an inverse, pre-order DLC packs for F 2nd not included in any of the current Japanese DLC packs are the Americana module for America and Europe versions, and the China module for China version.
Averted as of February 2015, where all foreign pre-order DLC costumes were added to Project Diva F 2nd in all territories.
Project Mirai Deluxe was meant to end this for 3DS owners by having the game come out in America and Europe a few days before the Japanese version, in late May 2015. However, the game was then delayed to September in these regions, while the Japanese version still released in May.
The English version of Project Diva X was developed alongside the updated PS4 version, thus avoiding this fate... in America. Neither the PS4 nor Vita versions have any plans to be released in Europe, so this is sadly still in effect for the series.
The music video for "The MMORPG Addict's Anthem" in Project Diva F takes place (and debuted) in Phantasy Star Online 2 and features Miku dressed as a FOnewearl. Project Diva F was released overseas in 2013, while PSO2's North American localization remains in Development Hell to this day.
The depressing thing being here that Phantasy Star Online 2 was known to have a english translation in 2012. SEGA apparently cancelled it due to the same cash shop issues that killed Phantasy Star Universe on XBOX 360.
Recursive Adaptation: The video for "Black★Rock Shooter" appears primarily inspired by the anime Black★Rock Shooter than the original PV itself (down to BRS's signature blue flame in her left eye and Black Rock Shooter's cameo), as well as the song being the version from the 2012 anime rather than the original. So it winds up being that Project DIVA F channeled the Black★Rock Shooter anime, which channeled the OVA, which channeled the original song.
Remade for the Export: Mirai DX was originally intended purely for non-Japanese audiences, being effectively just mirai 2 with one extra song and a few bonuses. Japanese fan demand for the Regional Bonuses caused it to be released in Japan as well (and, ironically, still before the English release, due to the latter getting delayed). Predictably, it didn't sell very well there, due to being little more than an Updated Re-release.