1) the avatar race means that the unit has a very high chance of defending. this makes Auric Ascended too vulnerable imho... his very high strength means that he will defend very often even without "improved chance to defend the stack"
2) I think it might be good to create a new promotion that has only one effect: making the unit "less likely to defend the stack" , and taking away that ability from channeling and hero promotions. this would allow to further customize units ( arcane units should all get decreased chance to defend stack, but NOT all units with the channeling promo should imho. vampires for example, and any unit that's good at fighting but can also use a couple spells. )
3) it'd be nice to have flexible difficulty raising/lowering difficulty less/more depending on the speed you're playing at.
4) you're already doing a great job at balancing the games at different speeds

I hope in the future you'll also have more stuff scaling with map size, that would be great. for example, movement points scaling with map size..
5) as MC said, having "impersonate leader" make you control both your civ and an AI one hotseat-style would be awesome. I think we can all agree that having the ai control your civ is not good
6) considering the amount of time and effort that goes into building Auric, the Meshabber and the mithril golem, I think it'd be very nice if they caused some havok around them if/when/where they die. the mithril golem could cause a huge earthquake that causes damage to non-flyers and alters the landscape ( randomly raising/lowering peaks and hills) , Auric could cause cold damage and turn the surrounding area to ice, and Meshabber could cause fire damage and start flames around. all of them should probably destroy improvements, and the concept could be expanded on other units, like Basium and Hyborem for example.
7) pyre zombies right now are very difficult to deal with, especially if your opponent does not rely entirely on them and keeps them protected. I think their explosion should not have a damage cap so that it can't kill units, and if they get attacked and successfully killed they should explode on the tile they're on and damage their own stack, making them as dangerous as they should
8) Succubi have mind 2 ( charm person ) , but they sonly have channeling 1 : shouldn't they have channeling 2 ?
9) axemen get a bonus against warriors, which is awesome. does that bonus also include warrior UUs like beastmen? it should, imho.
10) there seems to be an issue in base FFH that makes high priests of winter unable to cast call blizzard. not sure if this is true for FF though, I haven't played them in a while..
11) it would be nice if the info that is shown when you're choosing a tech would only list things that you can actually get. like, hiding heroes belonging to other races, promotions that you'll never be able to get, etc.
12) I know you've heard it many times already: when adaptive trait prompts you to choose a trait, it would be great to have mouseover tell you what that trait actually does
13) when trading maps, it'd be great if the game would tell you how many tiles that map would reveal, so that you could evaluate if buying that map/selling your map is a good idea or a ripoff before it's too late
14) I think MC's version of feast ( you can feast a city to death ) would be nice. of course, caution would be needed so that the dumb AI doesn't destroy its own cities
15) this mod (
http://forums.civfanatics.com/showthread.php?t=163657 ) has some VERY intersting stuff imho. like stacking food/hammers in a treasury that cities will be able to use later, for example. some mechanics that could work great with implementing food trade
16) AbandonRazeDemolish would be great to have (
http://forums.civfanatics.com/showthread.php?t=252243&highlight=abandon ) , being able to raze/abandon cities and demolish buildings can sometimes be very useful. in FF though, not all civs should be able to do all of this imho ( possibly based not only on civ, but on alignment as well

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