This is maybe the first genuinely sensible suggestion for a variation that I have seen in a long time. It would certainly be a game-changer, as when deciding whether to castle one's king into an open file, much rests on whether or not the file can be adequately defended to pinning or mating attacks without losing tempi, at least it does at my level... For that reason, it would probably only benefit the defender and improve middlegame king safety, which one might say would lead to more draws over time. However, if one's opponent's attack is based on a double-edged book opening that offers chances along that opening file with opposite side castling, then it would swing the game very much in the quuenside-castled defender's favour. The open c-file motif is with hanging a-pawn is a common theme of many positions, and I see that castling queenside with the king going to b2/b8 would nullify, or at least reduce the tension of a lot of these.
Does anyone a lot better than me have any other thoughts?
Imagine if it would be possible to, optionally, make a three square jump with the king, when castling. Arguably, this would enlarge the opening tree, as many more opening variations would become practicable. The long king jump could prove quite useful in most variants with queenside castle. At kingside castle, it is sometimes worthwhile to place the king on a more protected square. This ought to be good, for instance, in King's gambit variations. More information, and Zillions program here:
http://mlwi.magix.net/bg/castlechess.htm
Play chess with extended castle, online, here:
http://www.chessvariants.org/index/msdisplay.php?itemid=MPcastlechess
/Mats