On today's Planet Bob, merger is seemingly more common among established alliances, instead of just recently established micros. The reason for this is simple entropy, despite the fact that I believe CN to have a decent amount of life left in it, the older alliances become, the more likely it is for them to go into decline, a state which often leads to merger. It takes a lot more effort to keep an alliance going for five years than it does for one year, so it is significantly less likely for an alliance to last for five than for one. Sometimes this decline leads to disbandment and migration (often referred to as annexation, a term I believe to be inaccurate as it implies that the disbanding alliance is not making the decision), in which an alliance decides to disband and to encourage its members to move to another specified alliance, often one with which the now disbanded alliance shared a close bond (one example being when TFD disbanded and most of its former members moved over to NATO).
The other option is merger. In a merger, two or more alliances cease to be and come together to form one big, new alliance. The main problem with mergers is that they often don't last as the cultures of the merged alliances often clash and don't work together as hoped. This leads to the splintering of the newly formed alliance. However, what if there was a way to get the best of both worlds: give the cultures of the alliances their own space so they may continue on, but also create a central authority to share the resources of each alliance to the benefit of all parties. This would especially be helpful in cases where three or more alliances want to come together as more alliances means more cultures to potentially clash.
Enter the concept of federation. In a federation, the previously independent, now constituent alliances maintain a level of sovereignty, but they also establish a central government to manage the entire federation, which can also have the power to constrain or limit the authority of the constituent alliances in some or all spheres by imposing federal policies, laws, and decisions. The only current example that even approximates a federation is the bloc Die Linke, comprised of the alliances LSF, UCR, and SWF. However, Die Linke is much more like a confederation than a federation. While the terms of its establishing treaty prohibit secession like a federation, unlike a federation its member alliances are still treated as fully sovereign entities. There is a central committee that must approve of bloc-wide treaties and of new new members of the bloc, but there is nothing to prevent an alliance from holding or signing treaties individually, so they can still carry on their own foreign affairs if they wish to. All the members of the bloc are also completely in control of their own internal affairs, and despite the fact that all members are members of all the member alliances, through the dual membership clause, members cannot vote on internal matters (anything that does not affect the bloc as a whole, presumably) outside of their own alliance, and nor can the central authority impose any sort of internal policies on the member alliances that would in any way supersede their own, individual internal policies and affairs.
This makes Die Linke a loose association rather than a strong association. If three or more alliances were to come together, instead of merging or forming a Die Linke-esque loose association, they could form a federation. In a federation of alliances, the constituent alliances would retain some sense of sovereignty, but there would be a central authority which would exercise full sovereignty on behalf of the federation. To illustrate how such a federation might work, here is a hypothetical example of one:
Obviously, there would be many more details that would need to be worked out, but this provides a basic framework under which such a federation could be formed. I apologize for the longer nature of this entry, but federation is a somewhat complicated idea and I wanted to do it justice.