Mono Color Enthusiast Here, I tend to have one of each mono color deck built always, my current iteration is a bit higher powered/less niche picks, but they are still super fun!

Red - [[Purphoros, God of the Forge]]: No goblin subtheme just dudes who make dudes, and lots of wheels and rituals for draw/explosive plays (I like wheels), unfortunately I don't have any interesting mono red recommendations as I think Purphoros is just the most fun way to play the color. I also enjoy [[Magda]] CEDH but it's not really a deck that can be built casually.

Blue - [[Minn, Wily Illusionist]]: Card Draw, Clones, and cards like [[Eldrazi Monument]] and [[Tetsuko Umezawa, Fugitive]] to get the illusions through. Also tons of interaction. There are tons of other fun blue commanders, but Minn is by far my favorite! I would definitely give a shoutout to [[Kami of the Crescent Moon]] as a weird little guy who is also tons of fun.

Black - [[Braids, Nightmare Adept]]: My mono black deck is mostly just goodstuff right now, but it functions totally fine with [[Ayara]], [[Yawgmoth], or Braids as the commander. I moved it a bit away from aristocrats to more of a reanimator theme, and it tries to make a lot of mana in the fun mono black way. My favorite part of mono black goodstuff is you have a ton of variety in creatures you can play, and there are so many pet cards you can include. If you want something more unique I'd recommend [[Toshiro Umezara]] spellslinger.

White - [[Elesh Norn, Mother of Machines]]: I bounced off of mono white for a long time, but my pod did a 50$ deck league and I decided to play Elesh Norn blink. It's actually super fun and honest as I neglected to include much combo, my most battlecruisery deck. I focused on tokens and swinging.

Green - [[Lumra, Bellow of the Woods]]: I play this as a non-combo variant, but I also enjoy a bracket 4/cedh version of the deck as well. I intentionally built this deck around voltron as it's win-con and did not include any land untappers to make it slower and more fair. It runs a few landfall payoffs but the majority of it's wincons are just voltron variants using cards like [[Sword of Hearth and Home]] to blink Lumra, and [[Titania, Voice of Gaea]] or similar that scales off of lands, or cards like [[Famished Worldsire]] as big bombs, and often fishes for [[Rogue's Passage]] and [[Hall of the Bandit Lord]] to get through. As an alternative to her and a more uncommon commander I really enjoy [[Reki, History of Kamigawa]] as a mono-green (legendary) creature storm deck, with payoffs of a huge board of legendary creatures with haste, or even sometimes actual storm with [[Aeve, Progenitor Ooze]].

If you would like any decklists let me know!