I'll give a full feedback on the game mechanics and elements. Of course everything is just my opinion and tied to my way of playing, even there's lots of things I will address here, I want to make clear I find the game great as a whole. For the reference I usually play on very hard or impossible, mostly against the AI (single player), and usually play with normal speed and settings.

Game pacing:

- This game, like many 4x games suffer the snowball effect: at first on higher difficulties you struggle to keep AI pace, but when you start to go up in some fields (specially money & influence), you quickly became an unstopable empire. The main stopppers, disapproval & influence, can be seasily gamed with some time, specially mid-game.

Money, money money...

- Money rules everything in the game. You will quickly assign the pop on gold with L1 Economy plan, as nearly doubles this resource. You will quickly aim for 'Prisoners, Slaves and Volunteers'. Economy has a dedicated empire plan branch, even other branches synergize with this one (Unkeep from military and reduced cost from buildings).

- One third of the luxuries will affect your economy greatly. Unkeep cuts, money output, better trade...

- Once you feel comfortable in other aspects (influence, research, military), is extremely easy to get an economic victory, due to the huge array of gold-related items/boosters in the game. This is specially true for the broken lords and the forgotten, that need to focus in dust, and is hard with them to NOT win by economic victory before you get to a expansion/supremacy victory.

The tech tree:

For an effective gameplay will focus on early era techs for FIDSI most of the time, so you spend most of the time researching prior to the current era techs. You usually want all FIDSI-related techs from Era I and II.

While later era buildings are usually more effective, the huge cost increase (add strategic resources to the math) usually doesn't make them a good option until you covered all the previous era versions.

I also feel you advance the eras too fast starting from Era III. This is mainly because you get your Era I and II techs faster and faster as you advance.

Most of the military-buildings are usually a waste: because your will give priority on fids, army size, and later strategic resource reladed items before, so there's little room for defensive techs (I guess they have more use against another player).

Ultimate research are killers, I think they are good as they are, too costly and quite powerful, with the exception of the Serum of Iteru research, that makes your units 4x more powerful, making the war game a joke once you research it, even against much more powerful enemies.

Buildings:

I feel there are many less useful buildings most of the time:

- Anything influence-related past public works from era 3 is usually a waste. You can easily get to L4 every empire plan without investing pop into incluence by the time you get National museum in every city plus the national arena.

- On industry-related, only Mill foundry and Apprenticeship Registry are trully good. Anything past these two are quite situational. Once you get access to Cargo Docks, if you have easy access to water tiles in the region is another great production booster.

- On food, I feel Cultivation and public granary are the clear winners, Plow Factory is also great if you want to have good pop values. Other buildings usually are not woth the effort.

- On gold related, everything is nice if you truly go for money. The worst building is the Dust Depository, but comes along with the great Dust Tranmutter, I feel the gold buildings are all quite useful.

- Science buildings are OK, some of them are situational but none of them is a total waste.

- Top tier happiness buildings are useless most of the time.

- All defensive buildings are a waste, I usually ignore them, AI isn't aggressive enough most of the time to force you to get defenses up.

Resource-related items:

- The Mithrite and Hiperium accesories cost way too much to be useful. If you manage to get L3 accesories, costs are absolutely prohibitive.

- Improved Life 3 for units is much better than any other option defense-related. 60% extra life wins over regen, defense, life drain... On top of that, doesn't use strategic resources. Coupled with the life insignia and some gold equipment gives your army insane health pools.

- Other than near useless titanium and glassteel weapons, resource weapons are way too powerful. Lots of attack added (100+ attack in Adamantian/Palladian L2), maybe too big attack values. Palladian weapons give lots of speed, while adamantian give a little extra defense that doesn't scale. Mithrite and Hyperium are not good enough and are harder to equip, only useful if you get similar levels to Adamantian/Palladian (Quest and luck based) due to the higher damage.

- The Talisman gives way too much movement, nearly a must for units with 4 base movement, it alows you to move with your army 75% greater distances if you have slow movement units.

Luxuries:

- Emeralds and pixie blood doesn't give great bonuses, because of the defensive nature. Emerald in particular is a waste of a luxury because it only give +5 happiness in top of the extra defense.

- Wine is a game-changer on higher difficulties, +30 happiness is a lot, it usually means +15% in all resources due to happines on its own.

- All the FIDSI related, are quite powerful, either 50% for one type of resource, or dust orchid that give 20% to four different resources. Maybe too powerful for a luxury.

- +100% unit exp stack somehow with trained unit initial exp, so having this luxury gives starting L8 instead of L6 units at Era VI. Too big benefit in my opition. +100% hero exp is also too powerful, at least doesn't affect initial hero values.

- All the gold-related bonuses are quite substancial on their own. Particulary -75% unkeep on heroes gives you the ability to have as much heroes as you want mid-game.

The marketplace:

The problem with luxuries are not the luxuries themselves, ok they have huge bonuses if you get a few you only have to adjust your tactic, but the ability to stack 12+ luxuries mid-game thanks to the marketplace is what makes luxuries way too powerful. They are much much better than advanced buildings mid-game.

Heroes:

You have access to random heroes for any faction, something that gives you too much flexibility choosing which heroes to add to your empire. On a mid to long term basis the faceless heroes are the best hands down. Nice percentage bonuses both for combat and city.

Basically all the hero hiring is too random, so is too much luck-based to get the heroes you want for your cities and armies.

Cold Operator skill is just too good, and kills the hero infantry skill ice in the veins, as is just next to it in order of progression if you choose the class path. This skill is a must for any city/army hero.

A few words on maintenance costs:

Unkeeps costs doesn't scake well in the mid game, moreover, you can positively ignore army maintenance when you get several gold boosts from luxuries. Not to mention you can reduce to 0 the army unkeep with the necrophages mid-game. Can be also done with the wonder Reliquary of Auriga late game.

You can also negate expansion disapproval, but you need 30 pop in one city and get the legendary deed of the 50% cut.

On the assimilated minor factions:

Silics is the clear choice, followed by Dorgeshi. All the rest give minor bonuses and depend extremely on luck to get a good quantity, I find it quite balanced when you don't get Silics / Dorgeshi in empire.

Some random suggestions:

This is what I would change to adjust the game more to my playing concepts (maybe I make a mod of the game with some this changes in the future, but the mod options are quite restrictuve).

- Luxuries toned down: +30% on FIDSI, +50% exp on units/heroes, wine +20 approval, 30% hero maintenance cut.

- More powerful advanced buildings for food/industry. Maybe a buffed version of the Mill Foundry somewhere in Era III, IV, advanced food buildings really give huge amounts of food to cover the exponential nature of pop (+4 on exploitation, +6 on terrain with food...).

- Mixing army benefits with defensive buildings, so you are forced to spend more research in army buffing. As example, you get +1 slot in army/garrison instead of +2, but mixed with a defensive/exp building.

- Maintenance of units scale a bit more, maybe adding some additional costs on L4+ units, or an inherent cost based on the nunmber or non-militia units.

- Old era techs count as 1/2 tech for the Era progression counter, and a minor adjustment on era researchs required to advance.

- Tone down advanced incluence/happiness buildings a bit so ultimate versions are useful.

- Mithrite and Hiperium accesories with similar cost values of adamantian/palladium (The strategic resources are rarer anyways).

- Either tone down attack values for S. resource weapons or up the values for the defensive gear. Give Mithrite and Hiperium weapons a little buff.

- Improved Life skill toned down to 10-15% life per level instead of 20%.

- Much less replenishment rate of luxuries in the market (maybe is affected with the AI bonuses on luxuries in higher difficulties?)

- Instead of 10 random heroes at your diposal, 5 random heroes of your faction and 5 random heroes of any other faction.

- Cold operator being less overpowered: only reducing attributes and maybe city penalties on FIDSI instead everything winter-related.

- Unkeep/Disapproval cuts doesn't add together, instead they apply multiplicatively: -50% and -50% would be -75% instead of -100%.

- Serum of Iteru give +50% health and damage instead of +100%.

Thanks for reading this bunch of text.