CoIntiDencesさんのイラストまとめ

The design of Hivolt from Mega Man ZX bears resemblance to Harpuia's Armed Phenomenon from from Mega Man Zero 2, notably resembling an aircraft with green-and-white wings. According to a data disk in Mega Man ZX Advent, Hivolt's DNA was also used to create the Pulse Cannon enemy.

Exclusively in Japan, there is an official light novel of Speed Power Gunbike, Inti Creates' first game. Here are scans of the colored illustrations from that book:

A few of these were changed for interesting reasons.

Carrera's original Skill was written in Kanji and roughly translates to "Super-Heavy Magnetic Exploding Star".

Isola's Skill was changed to "Wonderful Rush" to reference the single of the same name from Love Live.

Azure Striker Gunvolt seemingly took inspiration from a plot-beat in Mega Man 10. In both games, when tragedy befalls the main supporting character, the protagonist storms the enemy stronghold in a rainy storm.

Character artwork of both Gunvolt and Beck, from Mighty Gunvolt and Mighty Gunvolt Burst. The original Mighty Gunvolt featured more of a chibi-look in its key artwork, with Burst using more standard proportions but at a smaller overall scare.

Artwork of Omega's second form from Mega Man Zero 3. The Complete Works book describes his appearance as a "combination" of Zero, X and the Dark Elf, which is shown by him wielding a massive Z-saber and X-buster.

This concept is similar to Biometal ZX from Mega Man ZX.

Mytyl in Azure Striker Gunvolt 2 has a particular bell in her hair. In Luminous Avenger iX, Lola explains that its akin to a family heirloom, passed down the Kamizonos for generations.

Interestingly, Gal*Gun Double Peace's Shinobu Kamizono also has the same bell in her hair.

Various early designs for GV from the Armed Blue (Azure Striker) Gunvolt Complete Works artbook. The green coat from his "casual attire" in the first image would later be reimagined for his everyday attire seen at the end of Azure Striker Gunvolt 2.

An early concept for Azure Striker Gunvolt 2 was for Copen to be the sole playable character, and for Gunvolt to be working with Sumeragi. The planned design's mask was used for the fake GV in Mighty Gunvolt Burst.

Now, GV is also allying with Sumeragi in Azure Striker Gunvolt 3

As a side series to the mainline Azure Striker Gunvolt series, Luminous Avenger iX adapts the 'naming formula' of the prior series in both its original and localized titles.

The localized formula, naturally, also applies to the 2015 April Fools 'game' Crimson Smasher Gunhammer