This is Work in Progress, I will update my thoughts later on.
The thing with the crusade is that only the first crusade was a 'non-political' crusade. After that, monarchs backed crusades. (
Second Crusade)
The Crusade System is in, it needs optimization and more love. This shouldn't be handled with a DLC, I don't want to pay for every feature which is in the game! I agree to pay for add ons (e.g. SoI, LoR), but I refuse to pay for improvements to content which is basicly already there; crusades for example.
This is a thing which should be handled through patches. Developers should improve content which is introduced by them, but they shouldn't charge it all the time...
What I would like to see:
1. Yes, it has been mentioned 5000 times: PAGANS! There are lots of which could be done here, for example give them a unique decisions, possibilites to convert to christianity et cetera
2. Republics. There are lots of possible features, for a) outsiders to interfere in their inner affairs and b) to play them yourself, dynastically. You could also consider creating possibilities to change from a monarchy to a republic and vice versa. Trading, unique privileges, liberty (more or less) and a special relationship with their liege were things which characterized cities during that time.
3. The Catholic Church. There are plenty of possibilities here, but I don't think a college of cardinals is a good way to get this into place. It'd be better to get yourself votes by Prince-Archbishops or anything like this, in exchange for making them nastier. Also Anti-Popes should receive some love to make them more unique. I don't like the thought of bishoprics to be playable though.
4. Mongols. There are things which could possibly be done about them, but that is least priority to me.
5. HRE. After the ERE received love, I would like to see improvements done about the HRE. Actually, the emperor title was something which originally the pope lend to someone (see Charlemagne), later in conjunction with the German Kings under the thought of 'translatio imperii'.
6. Carolingians, other starting dates. A very interesting period, if you think about the several splits of the Frankish Empire. There are possibilities for reuniting the Frankish Empire aswell as a story-line which features the splitting and thus the de facto and de jure creation of the HRE, the eastern frankish regnum, and france, the western frankish regnum.
Things which could be implented in conjunction with a DLC, but should be imo for free:
1. Earlier Bookmarks. The dawn of feudalism was way earlier then Hastings, so I would like to see some.
2. Improvements to the feudal system in conjunction with the earlier bookmarks. Fiefs were originally not inheritable, so there could be things done about, that could also improve the difficulty.
3. Improvements to the faction system which is rather dull at the moment.
4. More Crown Laws. I think of possibilities you had in the first Crusader Kings with several laws set by the liege giving your realm an unique touch. However, there was room for improvement there, the AI acted not that 'intelligent' or responded to actions which you did as king.
This is it for the moment, I hope you read my thoughts and like it.