Note: for those that don't want to read my wandering nattering, here's the summary post.
1. Do the little things right. The big things you're taking on don't matter.
2. Graphical power is the fourth or fifth most important thing at best in a HOMM game.
3. Don't leave the campaign maps until last. If you can't ship with brilliant maps, it's not done.
4. Did I mention the little details? That's where you score a 95 or a 75 with a niche game.
5. Dont' reinvent the wheel. Make the wheel again but reinvent the parts that matter. Maps, Campaign, Music. Art changes, but if you don't really get what I'm saying, go back and play HOMM2 again.
Did you notice that 1, 2, 3, 4, and 5 are all essentially, don't blow the little things in different forms? Good. But, remember, if you blow the maps, you blew everything.
Shorter version: You're not going to vastly improve HOMM from where the series was at Heroes3. Accept that and understand what a heroes player wants. Brilliantly balanced casltes/creatures and maps.
Long blustery version:
1. HOMM is at heart, a little things game. HOMM stuck it's fangs into many of us because it had the perfect mix of detailed maps, character development, town building, combat, magic, quirky art and, above all, brilliant map design.
2. Style matters, but that doesn't mean graphical power. Making a game look good is great. But, don't put in a single thing that adds a moment of lag on a middle of the road gaming machine. It's the "style" that wins, not the splendor. HOMM2 is still the best game from an artistic standpoint for it's time.
3. Don't forget the campaign. All that work you just put into the latest version of that brilliant real physics engine (exaggeration) is wasted if you don't nail the maps AND the campaign. That hurt Heroes 3 to an extent. It had a horrendous campaign after Heroes 2's brilliant campaigns. In the end, for all the great touches, it has to be compelling. On impossible, only the best players should win the first couple tries on a map. Don't build something brilliant and hope the community fleshes out a weak map pack. Don't promise a later release of a map pack. Ship with it, or stay home.
4. It's in the details. Heroes is the little things. The sound the halflings made when they bit it, the many and appropriate music themes, the spell effects that didn't slow combat down to a crawl but were still great, the balance between very different unit/castle types, and a lot more. But, none of it was THE feature that sold the game (except the map balancing). It just all added to MUCH more than the sum of it's parts. This leads us to what I want....
5. Realize that you don't try to dress up a simple recipe just because you can. The people that love and buy this game are those that love it for the essence of what it is. You could stip out 3D maps, 15 second spell animations that become boring quickly, and anything else that just gets in the way and you'd be a genius.