I want to focus on the economic side as a player (and, of course, vanilla is terrible about this), and I'm a relevant developer. What ideas, specifically, did you have in mind for the economic system / buildings? I'm not familiar with Magnate Lords or Master Builders. How would you like to see things changed, if possible? There is only so much we can do that remains a lightweight and clean mod over vanilla (or, honestly, even if we made economy our #1 priority simply due to the very static economic system in CKII, one of the game's only critical flaws IMO), but I wouldn't know the difference until you spelled-out some suggestions.
The ideas I was considering are quite simple, since they are quite general:
-Improve the localisation of building lines. Vanilla made a great step back in inmersion introduciong the generic localisation (fortification I, fortification II) instead of the "flavoured" one (castle moot, arrow slits, etc). Reverting back to it, or better yet, making the localisation of building culture- or religiom-dependent would be great. Little flavour bits aid a lot, believe me.
-Improve the sense of unity of developement of a holding. If some buildings required other from other lines to be built as a prerrequisite, it would feel like you are building parts of a bigger whole, not independent things. For example, make it so that you need some levels of castle town to have acce to the high-end levys buildings (in helps inmersion if you only can have the best armies in the most populated or prosperous paces). -It is already implemented in one case: each level of castle keep need a level of castle wall. It should be generalised, in my opinion.
-Improve the sense of specialisation of a holding: make at least the latest upgrades of several lines mutually exclusive, so that it introduces a feeling of "limit of resources". Say, you cannot have full fleet capacity and full cavalry capacity, so you have to plan ahead. In that way, you also improve the diversifications bethween diferent holdings: you could have one dedicated to taxes and fleet, one to cavalry, one to light troops, etc. It could also more directly guide the ai building cohorently.
-Apply the two previous ideas at a province-level. It make sense, for example that a lonely monastery cannot develop its full potential, but it could if it has a flourishing city in the surroundings. Conversely, it makes sense that the first cathedral or university in a province (or even in a duchy) can develop its full potential, but subsequent ones are limited, since its social function, so to say, is already covered by the first one.
-(A personal favourite, but maybe out of place) Increase the variety of buildings, reducing their effects, for flavour's sake. (Think at the buildings tabs in the Elder Scrolls.... So many posibilities....). For example, having four lines for fortifications: main keep, walls, towers and gates, who are mutually dependent.
As I said, these are very generic principles, but I thinks that if properly apply theioy could explonentially amplify the sense of inmersion and setretegic importance of building-up your holdings, in the mood initiated by project balance levy/taxes tradeoff.