Hello and welcome!
Well, beyond the initial thoughts and the general brainstorming, the project has been sitting for awhile.
We have an excel sheet which we call "Provinces and Holdings" which is a database that gives an instant overview of regency and tax income, as well as holding spread and ownership.
When we plan a new turn, we do so in a second excel document called a "Domain Order", where the players, for a single specific document, enter the actions they are performing that turn.
This is then submitted to the GM for processing and implementing into next turns Provinces and Holdings.
The most obvious task for developing would be creating the required tables to contain these two documents and merge new turn orders automatically with the world document.
Graphics and world maps, army moves etc. is secondary to this core functionality, at least in my mind.
For the GM, planning turns for the NPC realms should be quick and easy, perhaps by a bulk editing module or through direct table access in a certain view.