A difficult situation but easier said than done sort of issue.
The easy route is to ensure the RP realms are connected. Why? The simplest way to put it is Roleplayers LOVE to have their names held onto, as original as possible with intent, if we decided to merge, we may LOSE our names because of it. Another is supporting addons to allow the flexibility to view other addons, such as TRP 3, XRP, and the likes. It isn’t perfect because the addon can sometimes view other profiles outside your realms but not consistently. And this is the BEST case solution.
If we decided we want to merge, we need to address what was somewhat stated above, need to a better way to secure names, obtain said names, or without needing to fight for said names. It may seem silly but some roleplayers will find it more appealing if you’re as original as possible (Not using characters with accents for example, á, é, í, ó, ú, ü, ñ). Sometimes some will even report you for this which in worst case scenario, using a whole name that doesn’t seem to represent a name at all, basically gibberish or “Leet” styled. It may seem ridiculous but it has happened… It may be exaggeration but names can mean a lot to roleplayers. Diablo 4 seemed to have solved this and should somehow work for WoW too. Now, another problem with Merged Realms is figuring out how to make it work with so many players without lagging each other out, there’s a reason shards exist but it doesn’t apply to RP realms for a reason too but if too many are merged, it becomes a lagging nightmare. On top of the issues outside of roleplaying is the legitimate gameplay, rare and super rare enemies become far more difficult to chase after.
If the support of only using two realms to link to another is allowed, then it would circumvent a lot of issues without needing to worry of others too, like names, fighting over rares, worry about lag/sharding. As I stated, its far more complicated despite many possible easy ways to go about it, again, why I think connecting/linking is the preferred method…