After creating the mod UnderSiege I have tried to create a stand-alone mod template based on the files I modified/copied to UnderSiege. That should allow you to bring most basic features of the Main campaign into your stand-alone mod.
1. Open the Divinity Engine and create a new mod without any dependencies.
2. Exit the editor when it requests to open/create a level.
3. Unpack the zip into the root of your Mod. That's in ...\Divinity - Original Sin\Data\Mods\<YourMod>\
4. After unpacking find a file "_MyMod_Main.txt" and rename it to _YourMod_main.txt (It's a good idea to have the 'underscore' in front of the name just in case)
5. Open the Divinity Engine. Load your Mod and create a level.
6. Add two characters called "Player1" and "Player2" into your level and make them Global and Check the "isPlayer" checkbox.
7. Add two items called "TeleporterPyramidA" and "TeleporterPyramidB" into your level and make them global.
8. Add a point trigger called "YouMod_Arrival" to your level and make it global.
9. Open Story Editor and move "_YourMod_Main" under "__Start"
10. Select "Generate Definitions" from File menu
11. Find the line "CharacterTeleportPartyToTrigger(TRIGGER_MyMod_Arrival,"");" from _YourMod_Main and edit the trigger name to match yours.
12. Build the scripts
Here is the Template:
Stand-Alone Mod Template