What are the 2 or 3 aspects of CK1 you wouldn't like to see changed (too much) ?
I'd say :
1. The tech spread system (this very limited control and the huge differences in tech levels between provinces of the same realm are just perfect for the time frame)
2. The medieval feel of character portraits (even if they may be changed a little I would hate with passion a switch to realistic colored 3d faces) and interface
3. The pre-defined kingdom/duchies areas (good both for immersion and gameplay : giving the conquest of some provinces a strategic interest according to the ones you currently own is far more interesting than allowing to create fantasy generic kingdom/duchies anywhere with just a number of provinces instead of specific ones)
I'd say :
1. The tech spread system (this very limited control and the huge differences in tech levels between provinces of the same realm are just perfect for the time frame)
2. The medieval feel of character portraits (even if they may be changed a little I would hate with passion a switch to realistic colored 3d faces) and interface
3. The pre-defined kingdom/duchies areas (good both for immersion and gameplay : giving the conquest of some provinces a strategic interest according to the ones you currently own is far more interesting than allowing to create fantasy generic kingdom/duchies anywhere with just a number of provinces instead of specific ones)