Ok, here it goes:
1. Separate orcs into city orc and stronghold orc culture. - According to lore city orcs live with other races in cities, and stronghold orcs are independent as we've seen in Skyrim.
2. All starting orc rulers aught to be stronghold orc tribal rulers. - An orsimer culture county represents a county where stronghold orcs are predominant population unlike what we've seen in Skyrim which is a few strongholds among Nords.
3. More orcish counties across Tamriel. Strongholds are all over the place, but let's say in some remote regions stronghold orcs are dominant population as I've said in 2.
4. Orcs can't form kingdoms except Orsinium. - Orsinium isn't one place, every new Orsinium is at different location. So let's simulate building Orsinium by making Orsinium a kingdom, and only kingdom that orcs can form. Orsinium should be harder to form than regular kingdom.
5. Orsinium is a city, and as I said there's a difference between city orcs and stronghold orcs. When Orsinium is built it's repopulated by orcs from all over Tamriel, many of them (or most?) city orcs. So once you form Orsinium your culture changes to city orc and you get new orcish government which has city as main holding, but it's still feudal. However, perhaps the vassals should stay stronghold orcs and you call them to war. City orc government can build special buildings giving more troops. This should altogether make you strong enough to defend from when everyone else attacks you.
6. Which should happen as everyone else should get destroy Orsinium CB.
7. Unless they become vassals of the Tamrielic empire, they should be able to exist under them.
That's my idea for orc mechanic.
Now some lesser flavor.
1. Orcs are mercenaries all over the place. Perhaps anyone could hire orc mercenary who should be better, but more expensive, than regular marshal for hire. There's also a penality for having wrong culture/religion councilor, so it's balanced out, and potentially giving the merc some flags which lead to good and bad events (other courtiers want to get rid of him, he kills someone etc)
2. Orc and non-orc become blood kin. An event, perhaps part of some adventure or pilgrimage or with your orc courtier whos life you save in battle, that either makes you friend with specific orc character or gives you orc courtier.
3. Playing orc should have a plenty of dead orcs all around from various duels. I'd include a duel in a bunch of events, from carousing to just being in same army. Honor and strength and stuff makes them like that.
4. A trait or modifier for dishonest orcs "breaker of code of Malacath". The ones who don't behave as they should.
5. Maybe it's too much to ask, as it would probably need a whole new mechanic, wood orcs of Valenwood? Or maybe not, it's just a stronghold orcs who live in Valenwood and followers of green pact. So one or two counties of orcs with wood orc culture in Valenwood. But I don't think they should be able to form Orsinium as mentioned above, but could have few special events/trait or whatever.
6. Similarly for iron orcs. At least unique culture?
7. Iron orcs tame trolls for war. A troll units and troll bodyguard for orcs? Also goblins and ogres were in Orsinium.
8. Trinimac religion or heresy?
9. Orc war paint. As trait or... well, something, it should be included.
http://www.uesp.net/wiki/Lore:The_Red_Paint