game design - Persistent elements of offline players interacting with online players

For a long time I was thinking about a quite abstract game design problem, to which I didn't see a good solution yet. In abstract: How can a player have owned bases, units and infrastructure in a persistent multiplayer world, which can be interacted or attacked by other players, all the while the owning player may be offline and unable to give his input?

As a concrete example:

Assume you have a very large 2D world, in which you can build bases and have units guarding them. The bases can be visited or used by other players as automated trading stations, in which you represent the vendors. But the bases should also be able to be contested by hostile players - be it plundering or outright destruction.

How can I solve the issue that the owner of the base may be offline during an attack? Or that a player attacks at times where people are usually not online (sleeping, working, school)? Or that a player may actively go or stay offline in order to exploit potential protection mechanisms? How can I avoid forcing players to go online for a certain time at certain time ranges in order to maintain such protection?

Ideas (which are not satisfactory though):

1. Bases cannot be attacked in normal ways. People can send "troops" with a delay hopefully long enough to ensure the other player can give his optional input. The outcome of these battles is an automatically calculated result. Such sieges may go over multiple days.


2. Bases can only be attacked while their owners are online - or have been offline for too long. The explanation why those cannot be attacked is severely lacking though - given that trading should still be possible.



3. When a player intends to attack, both players somehow have to make an appointment in order for both to be online and to start the battle. This is a bad solution for obvious reasons.


4. Bases are inactive and invulnerable while their owners are offline or setting it inactive, and trading/other activities continue only temporarily based on the previous online/active time. This raises issues with visible bases and ongoing infrastructure, as they'd essentially need to be hidden or hideable, and other players may get bothered by it becoming inaccessible.

What other methodology does exist or could be employed in order to tackle this online-offline issue with persistent elements? I am also open for mathematical or systematic solutions. Be reminded that other players should be able to assist either side as well, and that their online/offline times would require some consideration as well.