Regarding variant rules, it feels like rather than give pages and pages of rules that only a small percentage of DMs might use, that they will reduce it down to perhaps two 'master' rules. DM Rulings (in the moment) and DM House Rules (permanent changes). Give a couple of play examples for both, emphasize that you can and should do what's fun for your table, and connect it to the multiverse.
For Example
DM Rulings - The DM can ignore RAW when the result seems nonsensical or just not fun, such as a PC who takes the hide action to gain the invisible condition and then dances in front of a guards face as they walk into the throne room to assassinate the evil king. If the player disagrees with the ruling, allow them to change their declared action that led to the moment.
DM House Rules - the DM can change any rule presented in these books. For example, if your table doesn't feel that combat is deadly enough, you could make death happen at 0 HP, give Exhaustion levels for failed death saves, and/or change resurrection rules to require an ability check or the completion of a quest to work. Once again, be sure to get buy in from your players that they also find these rules fun and let them know they can be changed if the result isn't what they were looking for. It may also be worth discussing with other DMs on forums such as enworld.org about changes you are thinking about before you take them to the group to see if other DMs have tried similar approaches.