I've played several games to completion now, on Normal/Challenging/Hard, Small/Medium/Large, some feedback for the team:
I realize it's early yet to be pushing hard against the AI with all the balance issues, but the victory I scored in the title pretty extreme - the only place I can go from there is Ridiculous AI, and I'm not sure it would matter. Finished this game with the Epic Quest victory.
Oh, and I was playing Lord Relias with some vague idea that his bonuses would help with a hero-centric attempt, but they turned out to be fairly irrelevant, as I didn't really do much questing past the very early game. Speaking of, his Heroic trait gives +100% Prestige from Trophies - I don't even know what a Trophy is 
Anyway, most of the major reasons this can be done are already being hashed over on the forums - Heroes in particular need some serious balance work, but the AI needs some tinkering as well.
I had three, and only three cities for the whole game, lost one of them several times (delaying its ascent to level 5 by hundreds of turns due to the current pop-wipe bug when capping a city)
Here's the primary gameplay mechanics/issues that allow this:
Heroes:
Heroes are far, far too powerful. Many discussions on the forums about this, I'm sure you are aware of this internally as well. They need to be tinkered with.
Biggest offenders:
- Fully shared XP
- Fallen heroes in winning fights take no wounds
- Insane Initiative
- No cap on mana spent per battle (AoW and MoM both have this, no idea why you don't copy this mechanic),
- No practical limit on accessory equipment (this was brought up as a major balance issue in WoM, why is it still present here?)
- A handful of vastly too-powerful traits (+stat per level being the biggest offender)
- Maul is too powerful (though I did not use/abuse it this game)
- You can carry, afaict, unlimited potions.
They're far from unsalvageable, they just need some adjustment.
Razing:
See this thread: https://forums.elementalgame.com/415762
One turn razing is flat out broken. I rampaged across the landscape sacking AI cities constantly. Only AIs on the opposite side of the map could build up with any meaningful progress, because by razing cities in one turn, I was destroying dozens or hundreds of turns of work instantly.
Another major issue with this playstyle - I could hole up in a city for a turn or two after I conquered it to heal to full, then raze it and move on. Thanks for the hospital AI!
Cloud Walk:
See this thread: https://forums.elementalgame.com/415764
Teleportation is broken. This was raised in WoM, but it's still here, and still broken.
As an additional suggestion to the City Gates in that post, I would consider adding Portals as wilderness objects - none in Tiny maps, maybe none in Small maps, but a few in Medium maps, and a couple in Large maps. These would be portal gates that would allow you to teleport across the map - probably heavily defended initially, perhaps paying a fee per use, perhaps having to fight magical creatures while in transit each trip, perhaps taking damage in transit. They would create interesting 'chokepoints', and allow traversal across the otherwise ridiculous spans of terrain on Large maps, without allowing instant player controlled teleportation.
Cloud Walk though... ignoring its raw power just with your cities, with allies, you can even drop outposts safely far away on the map as cloud walk targets, giving you a personal teleport network that's totally cheesy. It needs to go 
AI:
The AI is actually in much, much better shape here than it was in WoM, but a few points -
All of the AI in every game I've played has been deficit spending. They seem to build up every structure in every city with no regard for their use.
They don't level their heroes efficiently. While my heroes were ~15, the highest enemy hero I encountered was maybe 7 or 8, not well outfitted, and with several companion heroes sporting battle wounds.
They didn't respect me as a threat - because I was sticking to three cities and not expanding, I was at the bottom of the leaderboard or at best in the middle (once I started mass-razing nearby opponents), so the 'winning' AI of my faction were content to ally with me and let me stomp everyone else. The highest enemy faction AI did go to war with me, but given the distances involved (no city gates, no map portals), they couldn't get an army across all the OTHER AI territory to reach me and counter-attack, while my guerilla army of heroes thrashed their cities to the ground with one turn razing.
Spells:
Spells are actually pretty well done, but I need to do a full separate post about magic in the game in general. In brief - the current spells are solid, but there are some mechanical/balance issues that cause problems.
- Mana usage in battle per hero is not constrained. AoM, MoM, and other games have done this - simply give heroes a cap on tactical mana that can be spent, adjust spell costs to compensate, and you prevent endless spellcasting of hyper-powered spells from the mid all the way to the end game (I actually really dislike mana as a mechanic, its too binary - its either infinite or not present, I'd rather see a cooldown system, but that's another post).
- Because Initiative is too powerful, Haste is far too powerful for its cost
- Multi-turn cast spells lose their speed disadvantage with high Init, allowing for turn-1 knockouts of entire enemy armies - only strong single figures or heroes survived this.
- AoE spells should probably not do figure-multiplied damage. I'd suggest giving some schools SINGLE target spells that do multi-figure damage, forcing you to make choices depending on the situation.
- Buff spells are generally good
- Debuff spells are generally not worth the time, largely because damage or buff spells are so much better. With rare exception, it makes more sense to power up your units than it does to debuff enemy units slightly. Gravemark is a good example of a strong curse. Mass Curse/Wither type spells would be good, but they need to scale with attributes/shards better, or buffs/damage spells need to be toned down. Chaos/Pandemonium are just stupid - I'm never going to use spells that can mass backfire on me. In general, debuffing enemy units instead of just killing them isn't worthwhile, so I'd vote for nastier debuffs. To compensate, give some curative spells instead of the near-useless counterspell line of spells.
- I definitely like the full-combat duration of buffs and debuffs. That alone makes them actually worth considering over damage spells. Gives buffs power and debuffs some bite.
- I really dislike binary resists. Some spells do have partial resist ability, I'd recommend doing something similar for binary debuff spells, where a successful resist leaves a weaker effect, but still does _something_. Otherwise I'm always going to use guaranteed damage or self-buffs over debuffs that potentially fail.
- I don't think debuffs show the % of success/failure? Should be shown, I appreciate the % chance to hit/avg damage display for melee/ranged.
- Shards are a poor method of powering up spells, and the Evoker line is too much of a crapshoot for powering up your caster heroes (great if you get it early, always have it later, but might be missing it in the early development of a caster hero, rendering their damage spells extremely weak). I'd recommend giving some thought to smoothing out the ramping of damage spells to keep them useful throughout the life of a hero and the duration of a game, but without such tremendous spikes in power.
- Overland 'strategic' level buff spells are a terrible, terrible investment of mana. Again, the whole mana system... well, that's another post.
Last thoughts:
Interestingly, I did _not_ abuse reloading, nor did I even lose my heroes past the very early game. I saved regularly to avoid crashes (which happened quite a bit), but once I got rolling, I never stopped, and I was rarely threatened. Even when I was, my full garrisons were enough to hold, and if they weren't, Cloud Walk would allow me to instantly respond to any serious threat.
My heroes were defeated *once* the entire game - the very last fight for the epic quest, largely because I didn't know what to expect and underestimated the pile of enemies (which reminds me, that quest needs to reset to full strength if you fail, otherwise you can just recover and go finish up the remnants - I'd also recommend having multiple epic quest finishes instead of the same one each game).
The AI had to deal with a rapidly moving pile of doom that could teleport away instantly and razed cities to the ground immediately, but I only had to protect one tiny corner of the map with three cities.
I didn't bother conquering either of the wildlands that I located, and my heroes were still wearing some _leather armor_ when I finished the game. I never researched metal armor for my troops either.
I largely rampaged on the strength of hero talents and a few key spells (Haste/Slow early, then nukes and some buffs later).
None of my heroes were super-powerful in melee until much later in the game, but they were resilient due to high dodge and hp, and later defense (my defense was actually pretty crap for the first third of the game, I didn't even have full sets of leather armor until probably half way through the game!).
More later.