Exactly, there's lots of fairly big regional airlines with expanding global reach, ripe for joining ST (or *A or OW).
For example, Alaska Air has strong and close partnerships with both Delta and AA. Sooner or later it should make up it's mind and join one alliance, though honestly for it having it both ways likely makes more sense: more feeder traffic, more award choices for MileagePlus members, so likely for it to join ST or OW it'd need some sort of a compensation for lost traffic & revenues by severing the other ties (as JAL was offered to move from OW to ST).
Then look at various worldwide strong regional airlines. Air Baltic (BT) for example. More than 60% of traffic out of RIX is BT. The fastest (and only) growing airport in the EU by far. Exclusive carrier of Finnish, Danish and now also German governments (though connections in RIX), more coverage in Northern Europe than any other airline (e.g. more cities in Finland than any other airline, though not sure if that is excluding Finnair), and one of the best and strongest coverages of Central Asia from Europe, together with very strong EU coverage and decent Middle East/Northern Africa coverage. Now partner with lots of airlines,
http://www.airbaltic.com/public/partners.html , for both codeshare & interline. But, would ultimately benefit by ST or *A or OW feeder traffic for its extensive routes to Central Asia from Europe and vice versa, just like CSA did with ST.
Really there's a number of good potential partners like that, but honestly many of them are now having it both ways by partnering with anyone who makes sense, so partnering with a big alliance also meaning to sever some of the other ties may result in a loss, at least short-term.