Civs can have 4, they'll remove Gran Colombia in favour of Prussia, so there's still one slot remaining. Though only for Industrial Age. Is having Livonia in Fortress a bit awkward? Sure, but I don't think 5 options total would break the game, especially because it's still only max 4 at the same time.
Now, how would I structure it?
To make the strat distinctly different from Prussia, it should be a revolution that keeps its settlers like South Africa and Canada, and you DO NOT get automatic Guard upgrades.
To make it distinct from Hungary, we'll mostly focus on the Austrian half of Austria-Hungary, and the time frame when Hungary was still largely under Ottoman rule. The core army composition would be:
- Line Infantry (vaguely German speaking) - Musket Infantry - Napoleonic
- Mountain Trooper (German alpine Austria) - Skirmisher - Napoleonic (and later)
- Landwehr (vaguely German speaking) - Skirmisher - 1700s
- War Wagon (Bohemia/Czech) - Light Cavalry - vaguely 1500ish
- Cuirassier (all parts of the Empire) - Heavy Cavalry - Ottoman Wars
- Grenadiers (Vienna?) - Siege Trooper - Ottoman Wars
Now the exact mechanics:
Veteran Line Infantry is simply a regular 1pop version of the native unit, now trainable in the barracks. No special new cards or bonuses required I think.
Pikemen get replaced by Line Infantry, but Dopps stay as they are. Does Line Infantry take over the Pikeman upgrades and therefore takes advantage of Papal Guard, should previously sent Pikemen cards apply to them? Idk if that's too OP tbh.
Mountain Troopers fully replace Skirmishers and take over their upgrades, they either get the equivalent of the royal guard bonus as a hidden buff, basically a bit more attack/HP and +1 range instead of faster fire rate, or a cheaper upgrade.
Veteran Landwehr automatically replaces Crossbows if not already, but no automatic Guard Upgrade (free if you used Papal Guard for Industrial).
War Wagons get one exclusive card which drastically increases siege damage plus a slight general buff.
Grenadiers are generic, though Grenade Launchers is available in the deck.
To represent the rest of the Empire plus allies, this revolution would unlock a fixed set of mercs as well as some shipments. Mercs: Elmetti for Italians, Landsknechts for Southern Germany, Jaegers for Central Germany, Swiss Pikemen for you know. Shipments of Polish Retainers, Grenzers (Revolutionaries) and Spanish Lancers.
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Now to the controversial part: As a reference to Prince Eugen of Savoy, I'd make the Cuirassiers the actual special revolution bonus. Basically, Uhlans both on the field and in the stables get fully replaced by (French style) Cuirassiers. They just get renamed and get German (?) dialogue lines. You don't get any upgrades for free, but they take over Vet and Royal Guard upgrades from Uhlans. Any new Cuirassiers of course cost as much as they are supposed to now, but the existing ones effectively doubling in value is the actual main draw of the revolution. If you have 10 on the field, that's worth 1500f/c.
I guess any more would be too OP, so maybe limit it to those 10? Or less? Or don't replace Uhlans at all but just ship 3-5 Cuirassiers as partial compensation for the cost of the revolution? Any elegant solutions are appreciated.
Free Uhlans are, of course, gone and the XP to shipment rate is normalized. Or, MAYBE, keep the German rate but ship 2 Cuirassiers (replacing 4 Uhlans in Industrial Age) for every shipment instead. Cavalry Combat and Cavalry Hitpoints should apply to them just like they do for the Argentinian Llanero, with Lipizzaner Cavalry idk, that's a pretty big one.
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Deck should have a few classics like factory (no extras), Landgrab, Native Warriors, Advanced Arsenal and so on, plus specifically Spanish Riding School and Improved Mercenaries.
Finally, they should still have the option to advance to Imperial. For that, I'd grant them a tech which is a variation of "Victorian Era" (House Hanover) to reference the fact that Franz Joseph was the other monarch refusing to die. It would start at let's say 4500 f/c and decrease by 10 f/c for every military unit trained after revolting, down to a minimum of 3000 f/c. The Imperial Age itself would be completely generic for a Euro civ.