I think instead of giving you specific deck ideas, I would encourage you to take a commander and/or strategy you enjoy and place particular challenging restrictions on yourself in order to stretch your deckbuilding skills and also to reduce your ceiling.

I've found that when I'm building casual, I attempt to do things that sound fun or silly (without being useless) and put all my resources into that. [[Grenzo, Havoc Raiser]], with almost every single [[Act of Treason]] style effect, has been hilarious because I use what my opponents have, I wanted to just run a ton of them because it was funny, and it allows me to accomplish my goal of "milling" people with red (and there are mill cards to support it). It's casual enough to be fun and enjoyable and also does steal quite a few games.

I would also set a lower price tag, in order to discourage you from using clear and obvious staples. 150-170 is a good range to provide you with some resources but to also challenge you to find things that people aren't using as much.

Finally, mono-color decks can be a blast and I might suggest looking for someone in your favorite color to helm it. I almost exclusively play mono-color decks and I love that I have a particular color that I have to study in very great depth in order to really find the card pool I need. For example, I play [[Myojin of Seeing Winds]] with Clones, I play Toolbox [[Multani]] with various "[do thing] equal to power" cards, etc etc, and I've found that they're very satisfying to build because I can do things people aren't doing as much and they have a much easier power level to modify as needed.