I wouldn't mind playing a republic in CKII if able. Although the numerous characters and dynasties in a republic in EUR tend to be a bit overwhelming for me, in a monarchy you focus mainly on your dynasty, while in a republic you have no core dynasty.
The point of CK is to play as a dynasty, not a place where there is no core dynasty. I was among those suggesting that it'd be a good idea to incorporate landless characters was playable at some point - if this is done, THEN playing as a Republic can fit the point of CK and still be allowable.
While EU3 is pretty generic, it is after all a game with global scope. CKII should be less generic for the sake of it smaller scope alone. There are also good less generic mods out there (MM comes in mind).
I'd love it if Paradox had the time, money and manpower to research and develop pagans, republics and muslims as well as they do for Christians, but they simply don't have enough time to properly represent everything at once. Again, EU3 pre-IN was, imo, an absolutely terrible game. It tried to do everything and failed at doing anything, which is why those expansions made it more interesting. Thus, I'd rather have the focus of the game be on medieval Christians and not the others.
You can't have medieval Christians in detailed way if the Muslims and Pagans are bland and uninteresting, as these were the external forces Christian lords battled with. You can't have detailed feudal lords, with basic and generic merchant republics, as these republic influenced in depth of the monarchies with their wealth alone if nothing else. Even in they are not playable, they should be pretty detailed, to properly “play” with the regular factions. How can you have a detailed medieval European Catholic countries without the Crusades? You need Muslims for that. How can you represent medieval northern HRE, Poland and the Baltic in general without properly represented Pagans? Hell, Venetian influence and trade reached every corner of Europe at the time. In the late Middle ages almost all Balkan monarchs were citizens of Dubrovnik (Ragusa), which acted as safe harbour if things got foul on the home front and moved appropriate amounts of money in that direction to provide for themselves in case they become political refugee. Here is a nice example of roleplaying and dynastic aspects which need a neutral merchant republic to be fleshed out. Bland and generic non-playable countries can only hurt the playable ones.
If you're saying you want a merchant republic to be fleshed out, nobody is saying otherwise. The issue is some people want them to be fleshed out
and so much so that they're completely playable at the level as the feudal realms. This is entirely different from having them function as a means to better represent the main demographic of the game

.
Pagans, Muslims and republics had dynasties too, and at a later date the Papal States too, in a way.
I'm not disputing this, but it's not the focus of the original game or this one. They only exist in this game to provide a better playing experience for your Christian lords.
That's cute. You just argued against something I never said. I said "the appropriate mechanics for the titles" and for CK that IS dynastic and character based. What I was saying was that the title of the game doesn't define what people are buying to play. Read what I actually said: "People buy Victoria to play the period with appropriate mechanics, not to play the United Kingdom". The game is undeniably named after a Brittish monarch, yet many players don't start a game as the already-mighty UK. "Crusader Kings" is the same thing, even if they had given the UK more features in Victoria.
What's cute is you completely misinterpreted what I said and constructed your own strawman. I said your point was veering dangerously close to that mindset and frankly whether you realised it or not, it is. You used Victoria 2 and EU3 as examples of playing "for the period, not the nation". This isn't the case with me or many other CK fans.
Not once have the developers claimed that this game is made for anything but feudal lords - not once did they claim the opposite for the original. Conversely, Vicky and Vicky 2 have always been about industrialization and rising liberal politics. Infact, Crusader Kings isn't even about the Crusades - so there's another "misleading" title.
Might I also add that I didn't really see anyone asking for monarchy's or the old orders to be better represented in Victoria 2. I didn't see Paradox even add some sort of basic java family tree in EU3 to show WHY you have no heirs to your throne (or why you do) and more importantly, why a succession war is occurring. When confronted about this, what did Paradox say? Well, simply as I recall it, "This isn't CK. We won't be expanding these features."
How can we play a game about industrialization when the reactionary's are so generic?
See above.
The Great