I've got some thoughts kind of like this for defense of regions. This particular idea I suspect would prove too confusing and frustrating for Lords ("Stop recruiting my militia, I'm going to lose my region!"), but I like the general thrust of it.
I was thinking something along the line of dedicating your existing RCs as militia.
This will make recruits of that RC become militia units while disabling dedicated RCs from the list of possible RCs where people can recruit from. So for players who want to recruit from those RCs, they will simply see not available or greyed out.
Recruits will directly become militias of the region. The region will have to pay them weekly like now. Each RC will generate 4 units (existing recruits divided by 4 - lv4 RC will generate 4 units of 50 men - depending on how you want to limit things, you can even put conversion rate such as only 80% of recruits being converted into militias etc for future balance). While RC is dedicated, RC will not be filled. Can't turn back into a normal RC for a month.
This of course will disable the current system of dropping your unit as militia since it doesn't rely on your units to become militias anymore. And also will disable being able to recruit from surrounding regions part as well. This will give a reason for even rural regions to have more RCs. To counter balance all the downsides of only being able to recruit militias from existing RCs, you can either drop the cost of building multiple RCs or you can allow even rural regions to have lv3 etc.
