Only 20 missions? Why not 120?
Ok. stop joking. Im sure you should start from single player missions - you can always merge it in campaign. 20 missions is huge number of scenarios to make!
Its not so easy to make interesting scenarios in later stage of campaign. Usually first missions are boring "spying type" and easy to making in editor.
My suggestions:
I would like to see hard combat in first mission - of course with good backround as text in briefing. Second mission should be more tougher - for example trying to reorganize allied forces after defeated battle. Third mission should be preparation for counterattack - for example by gathering informations (not as always in first mission) and last one should be decissive battle. No need to make 20 missions where half of it are boring pretending whole in the water.
Btw if you release missions from campaign as single one, players will give you feedback or suggestions - thing very important for your motivation!
Please to reconsider your idea for campaign
edit:
Can you show your approach trigger?