What I've done:

Challenging difficulty.

I create my sovereign. She's based on a character from another game, which serves as an interesting stress test on the versatility of the sovereign system.

Alright, so her basic fighting style, is that she uses dark magic to consume her enemy's souls. The magic renders her invisible, and slowly kills everything around her. She's also a skilled scientist and an expert tactician. She can't control her magic well and sometimes injures her allies.

So, Soul Eating staff, Level 2 Death magic, Tactician, Attunement, Brilliant, Scholar, Evoker, Clumsy. Custom faction with Weak, Death Worship, Heroic and Adept. Her fighting style almost exactly matches the source.

The sovereign also matches the source in that she's practically unbeatable. Using Dirge of Ceresa I can kill masses of enemies to produce however much mana as I want, then I use death mark to take down anything scary. Every level up I take (in order of priority) +initiative, -spell cost, +spell damage, +intellect, everything else.

I tended to get three champions that stayed with my sovereign at all times and specialized as powerful melee units. My melee forces stayed on the defensive bent, leaving it to my sovereign to kill everything. However, I took anything that improved my critical's damage when I could. My other champions soon found it difficult to do real damage, but I generally found some poison items which were sufficient to away with most opposition. Defense scaled up better (likely because that's what I spent my traits on.) and my other champions served generally as a shield wall for my sovereign.

I really had no use for cities, so I eventually stopped using them. Seriously. They're worthless. Maybe if I played differently. Even so, for now, the amount of time it takes to win, is equal to the time it takes to find and march to the location of each enemy. I have no use for gold, or research. Only traits are useful.

Despite having one army of four units, even the computer recognized how much stronger I was than them. So, I was always the one to declare war. The computer would eventually create somewhat fancy armies, but Ceresa's Dirge still basically one shot everything in the game.

Bugs (?)

Are air elemental scrolls supposed to be indefinitely reusable? Other scrolls don't work like that.

Corruption. Death shards. I can't use them. I can only destroy shards.

Barbs, the troll shaman spell. It stops tactical battle and forces me to use auto resolve.

I got a trophy from some guy in a desert filled with lost knowledge. It decreased my city's gold output by 100%.

I got a quest to retrieve a staff and save some girl from a prison of ice. After going to the place where I was supposed to get said staff, I didn't receive a staff for winning the battle. The quest was, therefore, impossible to complete

At times, I couldn't cast a spell even though I had enough mana to do so. It seems that, the spell requires not that I have the amount of mana it actually costs to cast it, but the amount of mana that the spell takes to cast without any equipment or traits.

I seem to crash a lot on turn one. If I reach turn two, it's fine.

Balance (?)

The staff of soul eating seems too powerful. I run around killing everything with Ceresa's Dirge, which basically produces infinite mana. Run into a weak wild area, one shot everything with Ceresa's Dirge before the enemy can move, repeat. I can produce a thousand mana in a few turns. Perhaps it would be more balanced if it could only bring your mana back to where you started the battle? I'm opposed to any mechanics that allow you to change the scale of objects, for just this sort of reason. IE, harvesting weak npcs that should at this point in the game have no impact, and using them to produce spells that can kill anything I look at.

With that said, Ceresa's Dirge might be too strong, but I really think the issue is Evoker, and my staff's soul eating. That said, I shouldn't be able to one shot entire troll armies without having control of a single shard. It's a damage over time effect! It shouldn't just be a Wrath of God card! Oh, and the computer seems to be unable to deal with me if I cast the dirge then run around the map avoiding combat while the poison kills everything.

Things that annoyed me:

Okay, so we have these special cool locations. As already noted, my sovereign was almighty. From around level 3 she can walk in and smite any of the special zones. With that said, the requirements to seize the areas are too vague. I was trying to capture a walled city place for instance, and even after clearing out every monster inside the walls, I hadn't taken it. There were also monsters from said area, outside the limits of the area! Besides having tons of monsters which each have to be hunted down and killed, the areas are filled with harsh terrain, so it takes ages to move around them, and scout them out. Was I supposed to raise a giant army and storm the place with lots of units? It seems like a single heroic sovereign would make more thematic sense. That said, I really feel each area should have some definite easy to find goal that opens the area up for use (IE, killing the area boss.). There could still be consequences to settling the zones without fully clearing them (namely, the monsters would attack you.) but it should be possible to do.

I kind of wish, in tactical combat, I could undo movement when I go somewhere that, in hindsight makes no sense. Since nothing has yet changed, moving a piece without acting should be like holding a chess piece without letting go. Would this be difficult to program? Is there some sort of ability that could abuse this?

The Adventure tree shouldn't have just been thrown into the magic tree. They're unrelated! I admit that they don't exactly fit anywhere else and a four item adventure tree doesn't make much sense. I'd place one adventure technology embedded into each of the three trees. That would be symmetric.

There should be a difficulty level where the computer plays to the best of its ability without cheating.

Enemies players often started very near to me. Besides that, I killed them instantly, as their armies, even while guarding towns, were no match for my ridiculous sovereign.

You shouldn't be allowed to try to cast on magic immune enemies. If there is no possible reason why you would want to do something, it shouldn't even be possible.

Outposts and pioneers in general were confusing. They still are, to a degree. The tutorial doesn't really help.

That said, so far what I've done doesn't really qualify as fun. Interesting yes. Just not fun. Maybe if I played in a manner more in line with sanity, rather than just playing to win.