I'm thinking the Welsh for the first non-Pope/Independent civ. I already have the art ready to go for them. To give them some field presence, I figure to separate them into two civs, the North Welsh (Gwynedd) led by Llywelyn Fawr ("Llywelyn the Great") and the South Welsh (Powys) led by Hywel Dda ("Hywel the Good") -- or one Welsh civ with both those LHs.
No Pope and no King to tithe/tax them, but access to the trade screens after a long hard struggle against waves of marauders.
In the case of the Welsh, we could say they need to generate X amount of YIELD_CROSSES (simulating converting the barbarian hordes to Christianity) before they have access to the trade screens -- So their economy type could be "Religion".
Also, to replace the Tithe/Tax, after they achieve the required CROSSES threshold, the invasion waves would maybe decrease, but never stop. I'd even throw in pirate ships of some sort to constantly prey on the water units. Even if this was only a pop-up during the off screen travel of the boat saying "Your ship was captured by pirates" or "Pirates took X amount of your ship's cargo". This same script could be run against all players, even to simulate bad weather at sea or whatever menace you want. I figure this approach would save on CPU usage more than having roaming water mobs to bog down the AI.
Maybe have some Robber Baron/Sea Pirate demand some amount of gold from them from time to time or the predation on their ships increases. If they pay, the Robber Baron gets set to a positive score, but let's say he has a short memory and growing hunger for gold until the Welsh finally have to "throw down" with him, then he never fully goes away, just goes away "for a while." So, instead of an all out war with the Papal Army, they have to knock heads with a smaller force every so often. Lets say the Robber Baron is a little A.D.D. and doesn't build up his force the way a Pope/King does. Every time he adds four units, they get pushed to the field, that way you simulate both the waves of invasions and the constant harassment by a Robber Baron.
For practical reasons, I don't know that I would let the Welsh have access to the galleon unit, that's a lot of cargo/investment to lose. But even though I would Land Start them, I would let them have access to building roads and coastal traders from the start. They are basically Roman citizens abandoned by the Empire, so they would have tech but not war knowledge.
Also, for historical purposes, I would replace UNIT_VETERAN with UNIT_MERCENARY and these I would make only available through purchase, never just showing up in the immigration queue. Also, I'd give mercenaries PROMOTION_AMPHIBIOUS and PROMOTION_GRENADIER1 since they are basically reformed Vikings/Pirate raiders, rather than experienced military units of the conventional sort. This would allow a player using the Welsh Civ units suited to "take back the land you stole" from the Anglo-Saxons, or whoever.
I like the idea of a global mod, everything coming and going is fine with me, I just don't want to break the period model of M:C :/
Edit: as an afterthought, I'm thinking that they should start with
monasticism as a starting tech and instead of a UNITCLASS_VETERAN and two peasants, I'd start them with a UNITCLASS_CHRISTIAN_MISSIONARY and two peasants. This would be in more in keeping with history, since they effectively taught everyone else monasticism as it is commonly understood today. The original Middle Eastern and North African version of monasticism is very different in character. (see
Simeon Stylites and
Anthony the Great for examples of monasticism before
St. Benedict of Nursia got hold of the idea, but even then, the Rule used in the Welsh monasteries was markedly different from that of Benedict, for instance, they didn't insist on celibacy or being unmarried to be a priest/abbot)