I would like to see simulation games which work together. For example, combining a transport simulator, a city simulator, a production simulator and a theme park simulator. You build a quarry which digs up clay, and another which produces coal, create bricks, use the bricks to build houses in a village, expand the village, find you need a glassworks, connect your villages together and handle trading between them with a transport system...

An idea I've had bouncing around for 20+ years was a variation on Transport Tycoon where you could "zoom in" on towns/farms and play them as if you were in Sim City/Sim Farm, or "zoom out" to connect to other regions run by other players, with trading or even combat. A universal format for "simulation stuff" would mean you could have it extensible, so you could have a dedicated farming simulator if you wanted, else it would be treated as a box with an average output, with the same logic applying to sub-elements, so if someone wants to micro-optimise a plowing algorithm, they can have a "plow management" simulator which plugs into the farming simulator, which plugs into the transport simulator. At the other extreme, someone who was only interested in country level trading could have the country outputs boxed based on an average level of output.

Might be too complex for any one player to handle everything, but even so!