Title? I don't need no stinkin' title.
I want something no one else I know does: A mass combat system that uses tons of miniatures and works with 5e magic.
This quote from the original PHB always captured my imagination:
The problem is - It's tough for me to imagine that a couple of 5e 6th level wizards could not wipe out a medieval-sized army.
Even the Battle System series came up with a different set of rules for magic used in warfare.
There is certainly no real incentive to make such a set of rules commercially. I also recognize the vast majority of D&D players would much prefer a narrative approach for mass combat.
So, it remains a pipedream of mine.
This was already answered in part by @rmcoen, wizards would just be countered by wizards. But I also think people overestimate the effectiveness of wizards. Medieval battles regularly had soldiers in the thousands, if not tens of thousands. A 6th level wizard can cast, what 3 fireballs per day? Using the TOTM guidance from the DMG, that's 25 soldiers per casting. Pretty minimal when considering overall expected casualties.
Add in the fact that fireball has a range of 150 ft and the caster has to have line of effect, whoever casts that fireball just put a big neon blinking sign on their head to the couple hundred archers on where to place their next volley. Even if they're at disadvantage to hit, that kind of concentrated fire is going to be difficult to deal with.
I also suspect there would be other counter-tactics as well. Whenever someone invents a tank, someone else invents an anti-tank weapon and tactics. A handful of wizards vs an army that has no experience with wizards would be devastating because of the fear factor alone. But in a world where wizards regularly go to battle? They may have an outsized impact but they aren't going to tip the balance of war.