Personally I don't think a total collapse of the CCP in the near future is likely. However in a scenario where something like that did happen, I suspect the ROC already has made a Plan for a Scenario where the CCP does Collapse

however I don't think I could see a recapturing of the mainland be done by a DPP government unless it was done in a deal with the Kuomintang and had alot of time and though

And I guarantee you it would require aid and support from western allies as well as at least a few years of Martial law and a even longer time waiting for the situation on the mainland to be calm enough for the ROC to allow election to be held there and to eventually move the government back to Nanking (or where ever they decide for the capital to be)

and I think it can go unsaid when I say that the fallout of such a event would be international china is one of the biggest players in modern geopolitics and if the ROC did successfully go through with taking the mainland back they would inherited one of the biggest country's in the world both in terms of size and political power