Can you guys explain in more detail why you want this and what problems it would solve? I can think of some pros and cons but I want to know what you are thinking and you have not given any reasons.
1. Marshalls will delay the army creation, which right now seems to be very rapid, being a small part of the reasons that cause total war. The delay is derived by the fact that only specific agents at a specific time in each faction will be able to gather a stong force and this will be further restricted by the power, personality, influence and location of the marshall. Some marshalls may be more defensive and will not attack and some other more agressive.
2.If the marshalls take decision based on their personallity, we may have some periods with strong offensive actions and some others with no expansion at all.
3.Army creation is not a simple decision in terms of history or roleplay. Creating an army should be an achievement for the player and it may create the feeling of gradually ascending the power rankings of a faction.
4.When being a king, having a marshall will ensure (possibly) that your lords will not declare random wars and they will always be forced to follow either your army or your marshall's army.
5. If marshalls are going to be a thing then the player as a king should be able to issue specific orders. This will help the player to focus on a specific playstyle (for example to defend more than attack) and it may offer a more strategic insight of dealing with sieges, intruders, or attacking (for example sent the marshall to besiege a castle, lure the AI to defend and then you as a player go and besiege another one, your real target)
6.If the system of rebellions is going to be applied to the near future, marshalls with high influence may act as unsurpers, thus forcing the player or the AI to frequently allocate the position to different people, thus ensuring that during a gameplay the player may experience different war-behaviours, thus not allowing every random lord to be a rebel- just be cause i hate you - giving a little bit more depth.
7.Marshalls can be used to calculate more complex decisions regarding peace and war declarations. This means that if the decision to declare war is bound only to faction strength or financial power - the marshall option can be a new variable.
So the "marshall" variable's conditions will add or change this outcome. Example: If a faction has a current marshal with high influence, defensive personallity and 3 fiefs with over 300 militias then there will be a x% chance (combined with the other factors) to affect positively a peace declaration to take place .
8. Overall marshalls may affect roleplay by making the king seem more like a king, and it may give a feeling of realism to the game.
These are just simple quick thoughts, i am not by any means a developer or someone with special knowledge on gaming design.