As far as themes go, what's currently rocking in my head is "Underdogs" and "Imperfect". Very similar to Persona 5 in that all of your characters are considered undesirables for one reason or another, but that doesn't mean you can't be heroic anyways. Again, I haven't really thought about it fully and hell, I might even lean more into the D&D side of things side theme I want to explore is "Simplifying reality to suit your fantasy".

Don't want to spoil too much, but the reason this is going to be a 5e game instead of a normal pbp RP is specifically because many of your enemies are basically going to be akin to DMs, players, and other folks who try to boil down complex social issues into black-and-white fantasies that suit their own ego's, with you, the players, being the "NPC's" who are meant to do nothing but serve as a strawman to their megalomania. Natch, y'all going to have to give them a reality check. So they become traditional enemies like Dragons, Beholders, Giants, Trolls, etc as their delusions overwhelm them and turn them into caricatures.

For example, a suitable miniboss would be some agro jock type who thinks their physical ability, on top of the merits and accomplishments they've earned from others who recognize and appreciate him, allows him to bully others he considers "weaker" because he basically becomes one of those "Might make rights" type. This in turn makes him into an Orc like enemy in the fantasy world with a lot of physical strength, flexing his might, but that's really all he has. Maybe he has barbarian levels, but he invests so much into strength he's easily outsmarted and as long as you don't let him use his strength, he lacks any other means of doing any real harm. 

That's just a hypothetical of course. Ideally whatever villain I do throw at you isn't quite so cookie cutter, but then again reality being both complex and nuance, while at the same time being simple and straightforward, is something of a theme for this RP as well.