Looking into your CCCube Thread, I am trying to find some suggestions that work with multiple themes in your Black section. From that thread, I assume the primary goal for this request is to strengthen Black's impact on your RB Wildfire Control Archetype.
A card that could work as a role-player in the more aggressive Aristocrats strategy you support as well as Wildfire (maybe?) is:
I see that you also previously supported a slight Zombie tribal payoff, and this guy could potentially assist that strategy if you ever feel like bringing it back. I love this little fella, and I think he would also satisfy your desire to leave value behind off of a resolved Wildfire.
A card that leaves value behind and would play well with your 'Swamps Matter' theme is one of my personal favorites:
Unfortunately it is somewhat of a 'nonbo' with your Devotion theme, and I also understand that trying to mix 'Swamps Matter' and Wildfire seems counterproductive. Both due to Wildfire requiring a multicolor deck, and with Wildfire blowing your Swamps (that matter) the hell out. In a Wildfire shell that manages to brings lands back to out-value the opponent over time, I wonder if it could work though...
As for the mutli-color part, I wonder if you could mix your mono-black theme with the 'feeling' of a Wildfire deck with the inclusion of:
As they can play quite similarly to one another.
And for my final suggestion, I wanna preface it with my realization that you include no Planewalkers. I understand that you most likely will not deviate from that design choice, and I wanna say I think that's totally respectable, but I thought I would include:
I think she would be rad in your list. She plays into devotion, swamps matter, gives you land fodder for discard, and can fetch by Land Type, which seems rare in your list. She sticks around after a Wildfire, and can help you dependably reassemble your mana base if you are going for the attrition angle. However, I feel like she is so fair. After a Wildfire, she doesn't have any modes of defending herself well, so other cards that lingered would be necessary for the best chance of success.