I'd swap Graceful Adept for Library of Leng. Cheaper, harder to remove, and added bonus against discard.

Cancel for any other counterspell. There are plenty of 1UU with upside such as personal favorite of mine, Admiral's Order. Rewind, Dissipate, Unwind and Negate are also solid budget options. Personally not a fan of Mana Leak-esk counters since if an opponent has a ton of Mana, they become useless. Also, Defense Grid is an odd choice for a blue deck since it hurts your own counters on your opponents turn.

I'd consider trying to find room for a card or cards like Coastal Piracy, Bident of Thassa, and The Indomitable.

Single protection to consider, Mithril Coat is amazing, Commander's Plate always pulls a ton of weight in mon color (may hurt commander's unblockable agaimst another monoblue but otherwise really helps), Dive Down not amazing, just a pet card when it was in standard.

I've had Kalonian Hydra win multiple games by himself. I'd agree with FormOverFunction in that I would consider cutting something else.

My favorite card is Grave Pact but that isn't quite niche. I don't seem to have as many oddball fun cards that I tend to play but there are a few oddball or less played ones at least.

Black: Koskun Falls is a favorite of mine. I prefer decks that do not attack, so the tapping is not a big downside. Tainted Remedy because it makes opponent's creatures with lifelink funny. Gift of Doom which doesn't use the stack when morphing which has come in handy more than once. Pestilence is one of my favorite black board wipes which typically is on my board with an indestructible creature. Dystopia since at least someone is playing Selesnya.

White: Peacekeeper because no means no. Aura of Silence use to be more niche but has seen some prominent reprints. Solid removal with a tax effect has tripped up more than one opponent. Mandate of Peace is a great fog and a counter spell in some situations. Underplayed.

Green: Hall of Gemstone for any monogreen deck. Had a blue player try to win on my endstep and about 5 minutes into their combo, I asked where there blue mana was coming from.

Orzhov: Batwing Brume has killed a few Kranko players for me and saved me several times.

Colorless: Null Brooch is just Isochron Scepter with Negate already attached. Arena of the Ancients is great for when my commander doesn't need to tap, or attack, or is played after this and has vigilance. Great card no one expects.

Red: Apocalypse or Jokulhaups for the hard reset. Stranglehold because blue players are the worst and tutoring takes too long.

Blue: Mana Vortex for the slow decent into hell.

I agree he's a bit too powerful. However, I do not think he is off the curve considering power creep. Commander games tend to end quicker now, so a 5 CMC Commander is harder to pull off. I feel the ward was to try and protect the players investment.

I don't look forward to playing against him and paying 5 mana for a Doom Blade. However, he still dies to board wipes and subsequently he needs board presence to get the pay off, so he's especially susceptible to wipes.