I realize harness racing is now in a survival mode, but if they ever were looking for a new market, the answer may be out west; Montana and Wyoming to be precise. It seems they already have chariot racing, where they race with two horses (actually thoroughbred and quarter horses) pulling the 'chariot'. According to an article I came across, chariot racing has been going on in the region since the 1940's as farmers would challenge each other in races (sounds familiar, doesn't it). The states have pari-mutuel racing with thoroughbreds, quarter horses, paints, appaloosas, and arabians already, albeit at small tracks and short meets.
Of course, before there is any expansion, standardbred interests would need a good business plan as the last thing we would want is another Prairie Meadows or Canterbury Downs situation occurring. But it seems to me, if people are already used to chariot racing (albeit without wagering), it would not be a stretch for them to accept regular standardbred racing; it seems to be a natural progression.