I’ve been in a long-running campaign in D&D for some time now (think over 10 years), and our DM is awesome. One of the things he does well is be open to discuss ambiguous rules and interpretations, but ultimately make a decision that we all have to adhere to. Because our sessions are strung out (once every 3-4 months) we can end up discussing the same rule more than once, but if he makes a ruling then that stands for the one session and that lets up focus on the action/story and enjoy ourselves rather than get bogged down talking about rules.
The DM also somehow manages to keep us on the edge of danger/death without tipping over, which keeps things tense but gives a great sense of achievement. Whether he fudges the dice or is just superhuman at picking adversaries I’m not sure - perhaps it’s the number of reinforcements that arrive (which is easy to control when they are arriving from off-scene), which seems to just about deplete our magic/abilities without being completely overwhelming.
There are also puzzles/cryptic rhymes/visions to interpret to give us clues on what to do or how to do it, and sometimes he can be a bit too cryptic for us, but usually will help us when we either go completely off track or start to think along the right lines.
I’m not sure if any of that is actually helpful, but I can ask where he gets inspiration or tips from. He’s been doing it a LONG time, but still impresses us.
The one thing we did recently is start new characters. Our old ones weren’t dead, but they were level 18+ and starting to get really complicated to play and their actions were almost demi-god level stuff. Going back to basics really invigorated our fun and made it a joy to play again. Our old characters are still ‘in play’, but more remotely and occasionally, just to keep the top level campaign moving.
There are lots of mythical references, particularly Norse - but also Greek, Egyptian and oriental. Adapting existing legends to your world can make a relatively easy but complex world of good vs evil, positive vs negative, elemental balance, or whatever takes your fancy.
Above all - have fun with friends and enjoy it!