Here is what I have in mind.
Factions available for financial investment would be Imperials, Stormcloaks, Thalmors and all the Jarls.
The UI would be similar to investment perk in the speech. You go to Jarls' steward and dialogue option will let you invest gold. If you click that, option will ask how much, akin to buying decorations for your home. Options would be...
1,000 gold for potion, with more expensive potions being available at 2,500 and 5,000. If AI can't use potion, 10,000 for weak restoration spell teaching session. If that breaks immersion too much, 25,000 to add battle-priest to patrol teams.
15,000/25,000/45,000 gold for a additional patrol groups. This would be represented by increasing the frequency of randomly generated patrol guards (you know, those soldiers in group of three walking around). First increment will pretty much guarantee that you will see a single group of patrol. Second making it two groups with third making it three.
50,000 gold for guards repeatable gear upgrade with maximum quality being flawless.
For imperials, you would talk to general Talius at Solitude, and for Stormcloaks, you would go to Ulfric at Windhelm. The investment options would be much more extensive for these two factions, as they are the main warring factions in Skyrim.
The UI would be bit more extensive here. You would be presented with a map of Skyrim and faction's respective camps/forts throughout Skyrim would be marked. To make an investment, click on the marker for a camp/fort, which will open up option similar to investment for Jarls. Also if you visit those camps and talk to the officer, he/she (I have never seen female officers yet) will also give you a basic fetch quest for various supplies of weapons/armor/potions, except it won't ask you to go to those caves for the 1,000th time

Here is the main awesomesauce part that should greatly enhance the civil war theme of Skyrim. Once you complete the upgrades and do all of the supply quests, the camp would be go into "offensive mode"--meaning, it'll start generating those generic soldiers to attack the nearest opposing faction's camp/fort!

If you invest on Thalmor, Talos comes in and one shot you FTW

I'm not sure if Orc strongholds should be included because they probably don't want any help from any outsiders.
I think this would open a huge option into roleplaying as well (merchant of death, anyone?

So what do you guys think?