So, as I'm a player just like everyone else I'm surely able to come up with ideas too, right? 
My idea is to benefit alliances, perhaps those that are larger. It would give power to Alliance Admins to create Events within the alliance of various size and degree, but have some means of regulating it so it's not abused.
The Idea
Allow players within an alliance to contribute to a 'token' - can be named whatever and would perhaps be generated by other players helping out on shared missions. Maybe a token per mission, per player helping for example. If Player A shares a mission and Player B and C help, 2 tokens earned. This would go to a pot within the alliance. The method of earning tokens could differ, open to ideas in that sense.
Once there is sufficient tokens in the pot, an Alliance Admin can then build a mission. It can be placed anywhere as a normal major mission would but be named whatever they wanted too. Then, using the tokens, the Admin can add what is required for it. So it could be the admin feels 10 fire trucks are required, that could be 50 tokens? Patients, police.. basically pick the requirements for the mission and add up the tokens. From the required units a credit value can be given to each player for contributing, as they would attending a normal major mission. It could also have a time frame option (Short, Medium and Long) which would be random amount of times in between a range (Short being 1 minute to 3 minutes, medium being 3 minutes to 10 minutes, long being 10 minutes to 30 minutes - as an example).
Obviously being able to create large missions will see a larger number of credits being obtained. The token system effectively stops this being abused or used too often. The harder your alliance works together helping each other out, the more tokens you get etc.
My further thoughts would be calculate how much each player contributes to the mission. If a player sends 50% of the resources required to complete the mission, they get 50% of the final money. I don't know how complex this would make the idea, however.
I've come up with this idea because the current major missions don't come across as being too major at all. I can deal with one by myself without any significant knock on effect on my general resourcing. It's only because I chose to that I invite others in my alliance to come and help that they actually do come to help. Being able to large scale it (or create lots of smaller majors using the same system) would make things more interesting in an alliance in my opinion.
By no means is this any idea that will happen, but I like to create interest before throwing anything forward.