Actually, I predict the PCA will (within one or two decades) absorb the EPC and what's left of the conservatives of the PCUSA. They will become the new "mainline" as the original mainline implodes by virtue of having literally no more people (to go with their clergy, bureaucracy, buildings and endowments). The stage is being set. The center is being softened up. There needs to be doctrinal and practical room to absorb churches with divergent norms. A decade ago, I though it might have happened by now (or there'd be 10-15 years to go). My original time-scale prediction (a quarter-century) could still come true; there's still time left; and (it nearly goes without saying) the moves we are seeing are exactly what needs to happen for these mergers to take place.
And there will continue to be the "lesser" outlier-denominations.
And there will continue to be the "lesser" outlier-denominations.