Asklemmy

Something like Rome: Total War, but at full historic scale and with a total commitment to realism. You would need to come up with some kind of time scaling to make it manageable, but battles should play out over the course of in-game hours or even days.

Lines of communication should be important to maintain, as you can only see what your general sees and everything else on your screen is the result of reports coming in from your scouts/skirmishers. Your orders get delivered via a combination of shouting, flag and horn signals, and messengers on horseback based on the complexity of the order and the distance to the unit you're sending it to - which of course means they can be intercepted in some cases.

The battles play out with a simulation of crowd dynamics, where casualties from weapons are pretty low but if you can cause a retreat you'll be trampling the other army to death, or if you hit an enemy unit from multiple sides at once you can potentially cause a crowd crush that makes them unable to effectively fight back.

Massive blocks of people moving around should kick us a huge dust cloud in their wake, making them easy to spot but obscuring what's behind them. A unit standing directly behind another unit should be hidden unless you have high-quality scouting in that area. Cavalry should almost never stop moving, since doing so is pretty much an instant death sentence, with light cavalry automatically circling and using hit-and-run tactics while heavy cav simply attempts to trample their way through whoever they're attacking.

The biggest piece that would need work is sieges. Sieges should take place primarily at an abstraction level that allows them to play out over in-game days, weeks, months or even years - but then when the action ramps up you can switch to the normal battle scale to cover moments of interest. Both players should be constantly engaging in building and tearing down fortifications - Alexander's causeway to Tyre literally caused that city to stop being on an island, and you the player should be able to build similar earthworks. Huge ramps up to the walls, a second set of walls around your own troops ala Alesia, capturing water sources, digging tunnels, dropping hungry bears in the tunnels, etc.

Every time your army is on the march it's should be like playing Oregon trail, where your main goal is preventing as many of your troops from dying of disease before the battle as possible. Scouts give you conflicting information about the enemy's size and location and you have to sort it all out, river crossings are an ordeal forcing you to build rafts or a bridge or just risk wading through, food relies on supply trains to the mother country and a lot of foraging (ie stealing from local farmers), non-allied cities that you come across will preemptively surrender if your force is large enough and send you aid, and so on.

I'm not sure if you can do a "grand campaign" with all of this detail, so I would start with just a few specific ones, Hannibal in Italy, Caesar in Gaul, etc. Each one only has a few battles and a lot of events between them, with alt-history that can occur based on your choices and how well you play.