When it comes right down to it, there aren’t that many hard-and-fast rules about how to enjoy Magic cards. There are lots of variables to play with to see if they create fun puzzles for you. For instance, I sometimes wonder how decks would be different if each player started with 21 life instead of 20.
All the rules about deck construction are there to foster a environment that is fun because a variety of strategies can be competitive, allowing each player to explore their very own theories about what they can do to compete. Magic’s designers have mostly tried to avoid scenarios where players could win simply by buying the best cards. But over time, strong cards can become expensive.
If you think getting beaten by expensive cards is annoying, you might enjoy Pauper, a format where your deck can only contain commons. Commons aren’t always worse than rares, they just tend to be less splashy. They are less situational, but can have basic synergy.
Another way to alleviate the influence of mythics is to play Singleton Standard, where a deck can include only one copy of each card. I like this format - lets get some games going in Brooklyn!
You could also create your own format. If it’s mostly just for your friends, you can go nuts, making whatever rules intrigue you (people do this frequently with Commander variants). If you want to share it with the wider community, try to be aware of exploits that could break the format. For instance, if your format was that every time you cast a spell that costs one mana, you draw a card, you might need to watch out for how dominant a red goblin strategy would become (because usually the mechanic that limits aggro is that it can't draw cards). Aggro red is one spoiler to look out for in designing new formats, among many others. In truth, you'll struggle to discover a rules modification that will result in strategic gameplay as balanced as Magic's sanctioned formats; but trying can help you uncover and understand the give-and-take that makes them work.
If you’re interested in heady deckbuilding, a fruitful option might be to pick some blocks from the past and institute a ban list. For instance, you could allow the Return to Ravnica, Theros, and Khans of Tarkir blocks, but not allow Gray Merchant of Asphodel, Sphinx’s Revelation, or Siege Rhino. That might be an intriguing way for your friends (or people on the Internet) to get some use out of older cards, relive the fun things about playing those blocks, and explore which strategies might be good in the absence of those dominant cards.
Another way to warp a “solved” format would be to allow a certain contingent of cards from a different era. For instance, I could allow each deck to include two cards (four of each one) from any set. Again, watch out for things like Lightning Bolt and Dark Ritual to come up big; but as long as the format isn’t dominated by things that aren’t fun, this could be a great way to enjoy Magic cards!