I think there are few tribes left who could actually benefit from a merge at this stage in development in the server. Perhaps in the future things might change, but as far as I can see, it would need to be two small elitest tribes merging who share borders.
Now there are no (powerful) tribes, who exist at the moment, in that situation.
IMP, PnP and Dr NO are all far too large in member base to be merging with anyone at the moment. Best thing for them to do is fight each other. Or in this case, NO and PnP, fight IMP
BCB and MPFC are "mergeable", but share no decent borders with each other or any appropriate merging partners.
For BCB, the only option would really be UWA, but UWA is currently too large (on members) to be merging. If they slimmed down, then perhaps, in the future a merge would be feasible, but still unlikely.
Fireborne is too headstrong of a leader to submit or even share with someone else. Possibly UWA slim down and merge into BCB if they face troubles with NO, then the new UWA/BCB tribe would likely go to war with NO.
Meanwhile, there are only two options for MPFC really. And thats the GA and PnP. Both currently unlikely. PnP having too many members and being somewhat anti merge, even though there is a friendly and healthy relationship between PnP and MPFC, its just not feasible at the moment, nor is there or would there be any need right now.
The GA are closer to a food source for MPFC, if MPFC decided to get fresh, so I can't see there being any gain in merging with the, unless they intended to turn things around and try and fight PnP.
In the west, possible merges are SWIFT into Virus, IMP or RDA. But thats about it.