Maybe you guys already decided on something, but what if you could like convert a ship. It is like selling or returning something to a store and getting store credit. You sell you ship back, and some of the wood can be used for another one

No new actors are needed for anything, it's just the present units doing or getting stuff done when garrisoned (unseen).   Simplest would be that it can only be attacked (but the process should be fast enough for this to have no relevant use case). It shouldn't be capturable or salvageable, unless there's a method to stop its ongoing destruction (implying complicating things).

Where do we get an arsonist actor with appropriate animation and a burning building animation?  Same question regarding dismantling of siege engines. Which actor should we use to illustrate someone working on the siege engine?  Frankly, I'd rather park these for a moment and instead generate a new mod version that provides the reduced amoutn of recovered resources and the slightly slower destruction. QUESTION: if destruction is not instantaneous but a process taking a bit of time, how would we deal woth an attack onto that object during that time? Is it not attackable while being destructed, or, probably better, could an attacking enemy grab part of the resources of the destructed building ? (sounds complicated but is probably an interesting gameplay aspect)  

Excellent work @nifa!!

Sure, it'd be all contemplated, just takes more time.   That "while" would only make the "ugly simultaneous collapse of everything" happen a bit later . That's the main reason to require an arsonist. To reduce micro to just one extra action no matter the case, Set on Fire could work like this: select (all) building(s) and (all) arsonist(s), and just press the Set on Fire button/hotkey. This should make the arsonist(s) move to the closest building(s), garrison, "start the fire" and ungarrison (and automatically repeat the process until all buildings in that initial selection are set on fire). The only difference with Self-Destruct would then be the addition of arsonist(s) to the building selection to be destroyed. Self-Destruct for organic units is ok as it is now.