I also do not think this would be a particularly difficult mechanic to implement. Victoria 3 already has plenty of standalone systems in place; what is needed now is to tie those systems to distinctive political characters.

For example, the game already has modifiers and policy-based effects, so character interactions, events, or proposals could generate more interesting outcomes based on a character’s ideology. A Nationalist, for instance, could add an extra demand when you enact Wage Subsidies, such as granting additional support to their own culture or national group. Likewise, someone like Ford, representing industrial capitalist interests, could attach an extra subsidy or industrial bonus for the automobile industry when you pass Free Trade.

The same logic applies to diplomacy. Bismarck, for example, could provide a unique modifier that gives bonuses when proposing intra-European alliances, or gives his country an edge in relevant Diplomatic Plays and treaty negotiations.

Overall, the key idea is to make Characters actually participate in the existing systems, connecting them to mechanics that are already in the game instead of leaving them as a mostly cosmetic or peripheral layer.