Babylon does not get a discount to investment (unless that changed recently), Babylon got almost twice as powerful investments.
As far to investing in units, that's definitely not vanilla enough for G's tastes, I can already hear him saying how it is impossible
As far as promotions go, what do we have to choose from?
Ignore ZoC
Extra movement and extra damage to wounded units
Extra CS on open fields
A bigger flankingbonus
Ignore terrain-based movement penalty
Other suggestions?
I bet investing in units is possible, or how else it is possible that buildings turned from instabuy to invested in CBP? Though I might be mistaken. Anyway, from your choices I dislike ignore ZoC (it'd be just like Mongolia and we're making Poland and not Finland UB here). The rest can be okay, though ignore terrain could be too good.
I've got several ideas. I'll start with the tough one.
First of all, give
UU Heavy Charge bonus damage regardless if the unit is moved or not, I bet AI can't use this trait anyway (even I had problems last time I played Poland 50 billion years ago) so why is something only a player could reliably use in there? Giving the damage regardless would help the AI (and player) out.
Anyway, (
UB) Ducal Stable is the same, also Mounted melee units from the city receive Piast Legacy. (-25 XP maybe?)
Piast Legacy - +50% to 100% (dunno) Experience gained.
unlocks following promotions to be picked or gives one after fighting or preferably grants a random untaken one every X levels (starting from 0 or X?). If pickable, they're probably too weak as it is now :
God, Honor, Fatherland - +10% CS against civs following other religions. +10% CS.
For our freedom and yours - Receive Influence for every unit killed 5/10 tiles away from a non-hostile city state and +10% CS in lands of allied civilisations and city-states. (I bet at least one part of it is impossible and it's very hard to make use of anyway so this one can probably go)
Golden Liberty - +4/6 Gold and Great Person points towards a random GP in the capital for every kill. +5%/10% CS if you have more Gold than the enemy (or more policies total than the enemy? Synergy with unchanged/restored-but-starting-from-medieval UA this way).
F
or Faith, King and Law - 5%/10% for every different policy tree or ideology tree your enemy has adopted that you didn't, up to ??%. (if too samey as GHF/boring, add sth else like +GG generation? Yields?)
Just made them up quickly to be as unique as possible while fitting the context or at least name, though while they are unique (besides GHF which is meant to be the reliable one) I bet Gazebo will say impossibru to them anyway because I've seen similar stuff, but nothing really like this.
Alternative, easy solution that doesn't include uniques and I like for simplicity:
UA - old one, but starts giving from medieval. Loses -Crime compared to now. Can't be stronger because DS will be stronger. Maybe change it's name too, dunno.
UB - same as now but grants
Heavy Charge.
Heavy Charge gives damage bonus regardless if unit moves as well as +1 Movement. Maybe +Flanking damage/Base CS on top of that too, I don't know how much attack damage HC gives. Seemed to be almost nothing last time I used WH.
Maybe something like +XP% too, but then it'd probably require a name change or something and it's probably unneeded. Anyway, take that promotion away from base Winged Hussar, give it to all Polish mounted/armoured stuff via Ducal Stable. Give WH something good to compensate losing out on an unique promotion, like another unique promotion or one/two more regular ones.
UU - I recommend free
+Charge I on top of
Formation I it has already, and
free Morale. This way, Poland will be unique in not being forced to build/recruit in one city if you want all your stuff to be in tip-top shape and didn't get Orders. Since Poland is meant to be all about versatility and freedom in how to proceed, this one just makes sense to me? Far from the best UUs, but versatile and with Heavy Charge it will be up there anyway.
Sure, Morale could be given to horses by the Ducal Stable instead, too, but it'd be cooler linked to an UU (especially for people who receive them from CSs when Poland is not in the game). WH could get +GG gen and/or +Quick Study too but probably instead of something. Being simple while getting a good Heavy Charge in this case could be cool but I dunno if others agree.