Judging by population numbers alone, it seems unlikely that my realm of Feathermoon will undergo connection to another realm, since it's solidly a medium-population realm that is within the top 20% of realms in terms of character population. However, realm population isn't necessarily the only consideration for Connected Realms. According to Blizzard's mini-FAQ (underline mine):
Q. Which realms will be connected as Connected Realms?
A. We haven’t decided yet which realms will be made part of a Connected Realm, or which other realms they’ll be connected to, though realms will only be connected to other realms of the same type (e.g. PvE to PvE, PvP to PvP, RP to RP). In the end, we’re considering a number of factors, including Battlegroup, population, and faction balance.
The faction balance portion of that statement is relevant, because it brings up the Illidan problem. Illidan is the second most populous realm with ~213,000 characters, but is also the second most faction-skewed realm with Horde characters comprising a full ~200,000 of that ~213,000 total (a ~15 Horde : 1 Alliance character ratio). So, what does Blizzard do in this case? Does Blizzard decide to ignore Illidan's extreme faction imbalance when it comes to connecting realms, or does Blizzard connect Illidan to one (or, likely, more) Alliance-heavy realms to mitigate the difference?
If Blizzard decides to use this opportunity of creating Connected Realms to try to solve Illidan's faction woes, then I'd expect Blizzard to aim for at least ~350,000 characters or so for Illidan's Connected Realm cluster, which is significantly larger than the previously-mentioned Tichondrius' ~235,000 characters. In such a scenario, this would mean that if Connected Realms are to be made roughly similar in size, then every realm will have to be a part of a Connected Realm.
I'm interested in seeing how Blizzard will decide to handle the Illidan issue - and if Illidan players will bombard Blizzard with "You were not prepared!" memes if Blizzard chooses not to address the faction imbalance with Connected Realms.