MMO would be great, global transport, economy and terraforming.
I like "Green Planet DLC" but it's overkill to terraform an entire planet single-handedly in my opinion.
Economy with more components and technology and the need for own production specialization and trade.
Global wonders like global transportation system e.g. intercontinental maglev or global unified power grid, global water supply. Wondes that can not be achieved alone, only together with great benefits for all.

Ok I'm dreaming too much, it's work for a completely new game.

But what kind of multiplayer are you thinking of?
In the context of "Space Race DLC" would be very limited interaction between different players.
You can trade, share technologies, steal technologies and drones, nothing more. Currently it would be a pretty boring multiplayer.

Maybe it makes sense to think about what new content and interaction possibilities can be realized in the context of "Space Race DLC".
Something like common market or common research?

If you have created new interaction possibilities then you can think about the multiplayer.

hello thanks for the answer I think that space race can be used for the multiplayer but we can use below and beyond and maybe colonize other point of the planet, have allies and enmies that get new fonction.
We can imagine that we have 2 player in one map who intecact postive or negative or 2player in different map that have more interaction not like space race for exemple in the event players can decided to help a person or not.
An other possibility is have one map for one player and this map are in the sid.
We can imagine a new country or a new captain or a new research specialised for trading

If somebody interested for help me in development you can contact me