Some thoughts that immediately come to mind:

• A way of getting even more creatures as Braids fodder is Bitterblossom. I highly recommend it. You may also consider rejiggering the land base to make Field of the Dead worthwhile: you have enough lands to make it work, but more options to make it more feasible may not be a bad idea. You could also use things like Reassembling Skeleton to feed Braids even more.
• I know a lot of people don't like tutors, but tutors here can be quite handy. Vampiric Tutor, Demonic Tutor, Beseech the Queen, and Diabolic Intent are all possibly worthwhile. You could also consider some deck-thinning lands like Prismatic Vista and Fabled Passage.
• Mana-rock-wise, Mox Amber might be worthwhile.
• I vote Contamination for any mono-black deck. It's a great way to lose friends (which might be the point).

Stealing things and caging is fun: I've seen a lot of degenerate things with Mairsil. That said, I have a few small suggestions:

• You mention lands are an issue and color-correcting early. Indeed, while this seems to be the case, I can suggest more multi-lands such as Reflecting Pool, Mana Confluence, City of Brass, and the new fetchable surveil lands from MKM (Thundering Falls, Raucous Theater, and Undercity Sewers), or the route of ultimate color-correcting with Chromatic Lantern. There are, of course, also some of the super-fast artifacts like Chrome Mox, Mox Diamond, Mox Amber, Mana Vault, and the like, but not sure what your budget is.
• A theft option I always run in every blue deck is Commandeer. Nobody sees it coming (or maybe they now do, especially when I sit at the table).
• I'm surprised that you don't have the cutest and cuddliest blink card in there: Displacer Kitten. I see it in your maybe list, but I highly recommend it in the main deck: it's incredibly easy to trigger and it enables so many fun shenanigans.
• Other assorted recommendations: Fact or Fiction, Storm the Vault  ^Flip (you have enough artifacts to make it work), Goldspan Dragon (cute and cuddly treasure accelerant), Memory Jar, Lim-Dul's Vault, Patron Wizard...

Mill is fun. Mill was one of my first loves (and still is!) for any sort of MTG deck. However, I see some options to further Traumatize your opponents:

• Fractured Sanity is fun for all opponents. Or more fun for you. Brain Freeze is another fun instant mill that I highly recommend.
• A big concern I have seeing this is that people use graveyards as much as other resources, so being prepared for that may help you out in the long run. You mention graveyard hate is meta-dependent... but, in my experience, graveyard shenanigans are everywhere. My recommendations are Grafdigger's Cage, Tale's End, Stifle, Trickbind, Time Stop, and Discontinuity as stops to things like, for example, Eldrazi graveyard triggers.
• If you want a classic monocolor strategy and if you have enough of them, it's not uncommon to replace all the basics with snow-covered basics and use Extraplanar Lens as an acceleration engine. For you, I think that may not be a bad idea.
• Other fun toys that synergize with one or more of your toys: Windfall, Memory Jar, Isochron Scepter, Forbid, Jace, Vryn's Prodigy  ^Flip, Commandeer, Sunder, Teferi's Puzzle Box...

Some small recommendations I can think of:

• You have a lot of drawing for all parties, but your opponents will also be discarding (quite likely, especially given the Memory Jar). I therefore recommend Megrim, which actually is the reason Memory Jar became the first emergency ban in MTG history. You could also run things like Animate Dead, Reanimate, and Necromancy to take advantage of loading up your opponents' graveyards. Of course, Rise of the Dark Realms is a nice showstopper, though it's quite pricey...
• Archfiend of Despair is a delightful combination of Wound Reflection (already recommended) as well as stopping lifegain. I love using it wherever I can. (It's also a demon, so it synergizes with one of your alternate win cons.)
• You could also consider going more of a snow-covered route, so replacing your basic swamps with the snow-covered kind and going with Extraplanar Lens to amplify your black mana. Crypt Ghast also is another mana accelerator.
• The Meathook Massacre and Massacre Wurm are also "board wipe with benefits" that I highly recommend. I also almost always play Kokusho, the Evening Star in every black deck I run. Archfiend of Despair could also be another fun option for removing things and punishing your opponents.
• Some of your artifacts and tutors could be better. Rather than Wayfarer's Bauble, I could suggest Expedition Map instead. Grim Tutor, Planar Bridge, and Beseech the Queen could also make for fun tutoring.
• I'm not sure if Gemstone Caverns and Mox Diamond are worth it here: sure, they're fast and get you more mana quickly (always a good thing), but the multicolored mana isn't so necessary. There are options I might prefer like Mox Amber, Mana Vault, Volrath's Stronghold...
• For a mono-black deck, Contamination might be a great way to lose all friends. I highly recommend it.