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(+2)

Here's some more advice, that I've gleaned from running a game with a lot of homebrew rules:

Don't be afraid to add rules temporarily 

Not all rules have to alter the entirety of 5e, some can be included just for a session or two. I adapted rules for B/X D&D used to simulate Mongolian wrestling bouts once. The original rules suggested using them as an alternative to combat, I used my 5e version for 1 session for a wrestling tournament. It went really well and provided a nice break from the normal flow of play. 

Write rules players can bend

A lot has been said about when a rule should be written and when you should rely on making rulings. When you write a detailed set of rules, it can take away some of the freedom to act in unpredictable and exciting ways. The combat rules 5e uses are a pretty good example of this, it feels like a player can only act within a set of predecided actions. So when you write rules make sure there's room, whether it is stated or implied, for players to bend them in ways which may require you to rely on rulings. 

(+1)

these are both very good points!