I've beaten Persona 5 and, with all the rumors swirling around about a rerelease of the game, it's no wonder my brain is on the depths of Mementos... right?
I'm not able to come up with an actual monster dick design, at least not one that would spoil parts of the game for folks who have yet to play it. >_> (I do not want to give anyone any spoilers!) But then it hit me: Color schemes! For those who haven't played, Mementos is a dungeon in the game that is constantly evolving as the story progresses; each section of Mementos has its own color scheme.
I have a photo of one of the floors of Mementos in my spoiler tag below!
► Show Spoiler
It'd be mostly a psychedelic color, like neon purples or glow-in-the-dark greens for example -- then black and red as highlights. It'd look amazing on Demogorgon, imo!
And I'm sure I'm not the only one imagining a Blaze in Velvet Room Blue and gold... right?

While I can't envision a metaverse monster dick design (Not for a shortage of reference materials or anything -- seriously, where does one even begin?!) I can imagine a design based on Mementos itself. (I don't think it's BD's style, though, based on what I've seen in their shop.) Or... maybe on one of the 'shapeless' shadows (like the one depicted in the above photo) could be a good reference? That being said... Persona 5 has tons of color combos to go off of! I'd love to see what BD would create based off of something like this.