Depending on where you want to move things, you might be able to remove the existing Wye fitting, and rotate it to come up on the other side of the joist. If the ideal location would be through the joist, then, as Terry said, reframing things can solve it. But, really need to know where you want it to offer a good solution (assuming there is one - good is relative!).
ALso note, that if it is a 15" rough-in, with some toilets, if you were to put in one designed for a 14" RI, it would then only sit an extra inch out from the wall with no plumbing changes. You can usually fudge the location of the toilet on the flange a bit, so you might gain another 1/4-3/8" or so back towards the wall. Some of the Toto toilets can do that...you'd want one that utilizes their UniFit adapter...you'd have to buy the 14" version...it comes only with the 12" one (no credit for returns).