I think the idea of adding rubble has its merits, but as just a challenge, it wouldn't add that much (it would be very similar to slow down mining speed, for most aspects), and for possible new uses, the game has enough variety for the moment (I am not against it, but I don't feel it's a pressing issue, right now). Changing radically the farming system, instead, would be definitely higher on a theoretical list of welcome changes/additions (even here, though, I don't think biology in the fantasy world of DF will resemble the one in our world, so how much it will need to be realistic will open up a debate). What I am trying to say, is that most will agree that this is a long-term goal, if it will ever be added to the game. As a side note, I don't think that adding rubble just for the challenge is good (I like it, if it's for the added realism of it, though), because if challenge is the point, we can add as much as we want, we can artificially create it, and so it's arguably a non-issue (Toady could add heart attacks, which are realistic, especially if we consider their lifestyle, or he could add Gods killing random people, which I would consider less realistic, but the result would be random deaths and more challange in both cases... And if you want to argue that you can't plan against those, I might tell you that we could get more control over their diet or build altars to counter this chance, so it would suddendly have some logistic aspect to it, if we added more). What I mean is, that such an addition isn't a priority, it's as challenging as anything else could be, and could be easily mimicked in its results by simply slowing down the mining (is that hardcoded?) but on the plus side is definitely more realistic (mass disappearing is a little jarring), and might give your idle dwarves a purpose in life (but that's only an issue because right now you simply get too many migrants). Sorry for being a little repetitive, I am not a native english speaker, so I am not sure I was able to fully convey my thoughts on this, and I didn't find a better way to word it.