The issue is that not all players, and indeed most players, do not have the mind-set to appreciate the dimplomatic and strategic side of the game. If the company is going to make a game, it cannot just fine-tune it for the small set of players who appreciate and long for the kind of dimplomacy and such as you describe, or else the game will not sell well enough to justify its cost.
With regards to Napoleonic warfare, the issue is that it is fun and cool to us, but, and this is a big but, the fighting would be very simple and crude in compare to the kind of action we see on Medieval or Rome. There are only 4 types of soldier you are likely to find, a musket-armed infantryman, light cavalry, heavy cavalry, and riflemen (This is not including the arabs and turks, but history shows that it is not smart to use medieval era Mamelukes against European volley-fire). The only major variation on this would be that the French would have the Imperial Guards, perhaps some Highlander infantry for the English )(which are again little more than slightly more aggressive musketeers), and then the tiny difference between the countries. French infantry were historically faster, more flexible, but less disciplined than Prussians, who were much slower, very rigid, and so on. Russians would have to have Cossacks, that is a given, but their infantry musketry was terrible, and their favoured tactic was to use massed bayonette charges, with obvious results. In short, I don't think players used to variety and complexity of Medieval and Roman warfare would be so quick to appreciate the simplicity of Napoleonic warfare. It would just too different for most players' tastes.
One thing, though, that I think would be a great help is to add in a climatization factor for troops. Soldiers would fight differently in certain terrains and climates. Instead of just saying "an armour of 5 makes you tired faster in the desert" a "poor in desert" marker on the unit would say that it is one point of speed slower, tired (at least) 25% faster, and minus 1 or more morale points. A unit that is "good in desert" would gain a plus 1 speed, tire 25% slower, and + 1 morale. Something like that would greatly improve the dynamics of the battlefield. Viking units in MTW or in RTW, Germanic tribesmen types, would be at home in winter, while other units would be cold, tired, and miserable. Those from the desert would REALLY miserable, cold, and tired. That way, you don't have Mamelukes from the deserts of Algeria or Carthaginians fighting as well in the snows of the Alps as they do in the sunny grasslands of central Italy.
Another feature I want is a "sneak" feature. Units that can conceal in the open can move slowly without being detected (or at least with little chance of it) so that ambushes can be mor readily controlled.