I would like to think that all Factories exist alongside each other in some multi-galactic construct spanning space and time, such that it would be possible to interact with one another through intergalactic battle warfare!
The goal would be to create and send military units of different types, tiers and scale to the coordinates of your choosing which is ultimately another player's factory.
I think travel could work in 2 ways:
1. the slow direct way if "travelling" through deep space however this would take much longer and there is a risk of not having enough fuel and you risk losing everything en-route!
2. through a wormhole much like Stargate, this would require massive amounts of research and a high cost to open each time, gate only stays open for a short duration and additional resources are required to keep it open for a longer amount of time, anything passing through when closing is immediately lost! Requires additional complexities to choose destination by solving some complex, logical and visual puzzles which would determine the coordinates, not directly able to go wherever you want but you can send scouts through to investigate and you can then decide if you want send the rest of your militia or find another target destination.
Upon contact your units would engage in real-time battle with the target's units, could be fully autonomous or player controlled, and after some time duelling, the outcome could be that if the local resident survived the attacked and damaged/disable/destroyed the attackers units they get to keep them and are either able to recycle or even hack/repair the units and use it themselves. If the attacker destroys/disables the target's units or retreats, it is possible to keep the wormhole open long enough and send units back with stolen resources using carriers / recovery units or go back the long way...