I don’t really find that anything is required for 5e to be the game I want to play, other than things in the realm of player options, and things that add some DM-side complexity that makes scenes more fun.
I really enjoy flanking, level 1 feat or supernatural gift, +1 stat when class grants a feat, and either rolling or using a slightly higher point buy, but none are super important. The level 1 feat is the most important of those, but I’d still enjoy the game without it.
It’s funny to me to compare this to 4e, my other favorite edition, where the good news was that I only wanted to change 3 things, and the bad news was those things were the basic PC math progression, monster HP, and to make most stacking bonuses no longer stack. An undertaking large enough I never actually did it.