Hmm, guess I will try to sum up my thoughts overall:
*Main Point:
-Basically a reoccuring theme for many changes: I don't know how creators imagine a 1920s Calradia, but I instantly imagine a story of endless total war that started right after the revolution. I can imagine Red Vaegirs trying to ignite the World Revolution, coming close to destroy Swadia, then being pushed back by new Swazi regime and White counter-revolution, then a massive setback, revolution vs counter-revolution everywhere, basically endless war until One to Unite them All appears. I am not sure if that conflicts with the mod-creators vision much, but I just see 1920s Calradia as a time of apocalypse. What could be done to reflect that in the game? Well, see the list for yourself!
*Map:
-Increased density of settlements and forts, to simulate population growth in Calradia over the course of seven centuries.
-Fancy tweaks to old Calradian names of locations to simulate development of languages. I always imagine 13th Century's Calradia names are (excluding territories outside of Empire) are based on old Imperial language, since all the people of Calradia became "Calradized" (like some barbarians became Roman when dealing with Rome). It is flavour mostly, but could give more of a modern feeling to Calradia IMHO.
-Since it feels like the main focus of the mod is Swadia vs Vaegirs (Nazi vs Commie), which I like personally, maybe Vaegirs could be given more Soviet-style territorial expansion? All that winter territories to the west could be made into some sort of Siberia, not everything has to be controlled by Red Vaegirs naturally, but maybe Whites could have some stronghold there.
*Factions:
-Khergit China is irksome a bit to me, but then it's authors vision and I guess there's nothing to say about it.
-New Albion / Balion are meant to be like America, but if that's true then shouldn't they be based on Calradian cultures? There's no chance this can be changed at this point, but I'm just thinking: wouldn't Calradian powers colonize local territories? Then a Revolution, and a reconcillation (like US and UK). But who could be potential colonizers? Nords and Rhodoks probably. It is too late to change mentioned two, but maybe, if there will be new overseas territories added, maybe they could be based on a existing powers? Vinlandia comes to mind as a post-Nordic state for example.
-This might be too much to ask but with increased amount of settlements we could have more local, secondary factions (White Vaegirs, some sort of Anarchist haven, minor states) around Calradia, but it might hurt game performance and therefore it might not be a good idea. Depends. I have a good PC, but used to have crap one and know how it can hurt some people
*Items:
-Nigh unbalanced, but I imagine it is a massive task to implement them all and balance well, so it's a matter of time really.
-Faction-based items in faction's towns, no doubt about it.
-I like the suggestion of making armors entirely a cosmetic choice.
-Big suggestion: I really feel like Calradia of 1923 should be a land of utter madness, like former Imperial Russia territories in late 1910s - early 1920s. In case of items it means drastically raised prices of goods and potentially bigger amount of money you earn, to simulate potential hyperinflation and economical misery ravaging Calradia.
*Bandits/Outlaws/Bad guys:
-More ! Map feels so empty at times

Some quick suggestions here:
-Calradia: Looters to Mafia, much more Deserters (Calradia in total disarray), mentioned Anarchist as an overall plague in Calradian territories. General change of <insert zone> Bandits to Anarchists could be nice. And each faction could get potential specific rebel group:
-Swadia: Red Army Faction (communist rebels), Monarchists.
-Vaegirs: White Vaegirs, Cossacks (both in the game already).
-Khergit: White Khergits naturally, and some traditional old Steppe clans fighting Communist order.
-Sarranid: Jihadists are in game, but Sarranid is Islamic state right? Depends if secular or not. If secular then jihadist, if not secular then...anti-jihadist lol. Also tribal lords of the desert.
-Rhodoks: What are they exactly, fascists? But if they're Italia-like then stronger some sort of Mafia would work.
-Nords: no idea. Honestly.
-Can't tell about new countries, I haven't been there yet.
-One super important thing, if it could be doable then that'd be fantastic: Making the outlaws aggressive or not depending on relation with a faction. For the start for example, White Vaegirs/Khergits should have good relations with Swazi so that they don't fight. Instead, maybe if Swadia and Vaegir get to fight, White Vaegir unit nearby would rush to Swadian aid? It's just example as White Vaegirs maybe will be their own faction, but you know what I mean.
-The only permamently aggressive force should be Looters-Mafia, new Deserters and Bandits-Anarchists. Local oppositions would have to be set up to fight against their ideological enemies only.
-Faction-based patrol forces would be cool too.
*Units:
-I can always appreciate foreign names but they can be confusing at times. I would make unit names English except of Swadia and Vaegirs, since they are the main actors of this mod and everyone recognizes German and Soviet names.
-I like how current troop-tree is compact and contains a few options to go with. It's fine, but probably requires rebalancing, which is a matter of time again. Maybe I could tinker with something on my own, it's basic kind of modding really.
-Maybe turn some of the strongest units to elite troops recruitable in forts only? Nordic snipers come to mind, everyone is complaining about them

Vaegirs could have Konarmia (legendary Soviet cavalry force), Swadia some sort of SS, Blackshirts for Rhodoks if they're really Fascist, modern version of Mamelukes for Sarranids, etc. etc. you know what the point is !!
-My idea about troop development: it should be easy to recruit tons of people to fight in the times when war is only way to success (but that could be solved with more settlements) and upkeep for a simplest troops should be low as it is now. Promotion costs could go up (inflation) but what is really important is upkeep. It's not hard to find a gun in this era - everyone has one - but ammo production is probably scarce, and people are extremely opportunistic during the apocalypse.
-Super important point: morale should be way harder to keep at Excellent. Lower down food bonuses? Or just make some quality foods more rare. Grain and bread should be easy to come by, but getting cheese or honey these days? Dude, that's a luxury.
*Companions & Characters
-Just rename some of them really and put into modern setting. Bloody Borcha in 20th century are you serious

-Wilhelm Kaiser as claimant for Swadia maybe?
-Some renaming for the lords? I felt like some of them had old names from Native, I might confuse it with other mod though (tried out lottsa of them recently).
*Misc
-PBoD would be just amazing for this kind of mod, but it's probably hard to implement this kind of thing on the fly in the mod.
-Can't think of anything else really.
Now, to the point, I have some basic experience with Warband modding (and quite a lot of experience from modding old Paradox Interactive games) and I could help with some simple coding. Don't have that much time during weeks, but I could work on some relatively simple stuff, or some of my propositions of you mod leader people like them. If not, maybe I will attempt making a little submod with minor changes later on, who knows?
Still, gotta say this mod alone got me back into Warband after quite a while, it's just basically one of the best ideas in history. I like seeing Calradia in different settings, but this is just so much different one cannot even get over it. Good job I say. : D