I'm really looking forward to seeing what you've come up with for the tactical combat.
I have one question though, about combat length. Average combat length really isn't very informative. How many battles are small skirmishes? How many are clashes been actual armies? How many include powerful creatures and/or sovereigns or champions on both sides?
An average combat length of 3 minute might mean that 50% of battles last 1 minute, 33% 3 minutes, and 17% ~ 9 minutes. Or it could mean 90% of combat lasts about 3 minutes, 5% lasts 1 minute, and 5% lasts 5 minutes, as two illustrative examples. To be honest, I think you should look to games like HoMM III for combat length.
Early battles in HoMM often last deceptively long (but not tediously long), but they were nonetheless engaging and interesting because they still often required you to think. They were much more intimate, and every number in each of your stacks was simply that much more vital. Then you get to the middle game, where battles range from trivial and fast to difficult and methodical. The easy battles in the mid-late game stage lasted mere seconds. The harder battles in the middle stage could last a good 10 minutes. The harder battles in the late stage could last twice that long.
...So basically, my long-winded point is that I hope easy, straightforward battles will be pretty short, while large battles between relatively equal opponents will take significantly longer (at least 10-20 minutes). I also hope that combat between more than 2 forces is still in, as well as huge armies. For me, the "promise" of The Battle of Five Armies-like events was the single biggest selling point of the game, so I'll be quite disappointed if those features got axed somewhere along the line... (Quotation marks just because I know that there's no such thing as promises during the development stage, at least as far as what features will and won't make it)
One other concern about the changes to the magic system. If shards are now used to increase your mana cap, it seems like the ability to play as a small empire wielding a big magical stick is going to be significantly diminished. And that magical ability (at least in terms of mana, if not sheer power) is going to be tied inextricably to empire size. After all, larger empires will generally have access to more shards, and will certainly be able to defend them easier.
The balance to this previously was, in part, the essence cost of reviving the land. If you wanted to build up a large empire, you would have to spend more essence unless you have the patience to wait until the revival spreads. Therefore, sovereigns with larger empires would typically have less essence, which would translate into less magical ability. Now that essence is no longer used for anything but magical purposes, that is no longer the case. In fact, correct me if I'm wrong, but as it stands the only thing essence is used for now is in determining your mana cap?That's pretty disappointing, as it takes essence from being a very finite, very precious resource that forces tough, meaningful decisions on you to be some throwaway stat...