I wouldn't say that Timur is better-known than Indonesia at all. Leaderwise, sure, Tamerlane is a key figure but most people would be unable to name a Majapahit ruler. And there's also the "fan pressure" theory which posits a place for Indonesia because there's a greater fan desire for it than Vietnam.
I don't find the CS argument compelling - it was also argued that Belgrade was gone based on Riga's inclusion, until Belgrade was spotted in a later shot. And if we do give it credence, I would still suggest Timur has the higher chance than Vietnam. Both Indonesia and north-central Asia are areas that are underrepresented in the "Civ Map", and both have definable roles as civs that at least to me seem less tenuous than a Vietnamese claim based on ideology.
Personally I would think that since Timur is better-known than any Majapahit leader - or Indonesian leader - for that matter, it gives the Timurids an edge over Indonesia. Civ 5 does place a lot of emphasis on leaders, after all. Anyhow, of all the three, Vietnam has the greatest public recognition if only because of the Vietnam War. (Not to mention given the many Vietnamese-Americans who are involved in Vietnamese-American media, it'd be hilariously easy to find a decent voice actor, though this isn't important)
Additionally, I don't think fan pressure amounts to as much as we'd think - otherwise we'd have seen Israel, several modern civs, and other commonly requested civs in by now. However, it may be a factor - when it is at all. I don't think it really amounts to much since there are several fan favorites anyways, and I doubt we can predict
which ones in particular Firaxis would choose. Essentially, that's why I'm basing my reasoning on the Sofia CS theory, which
is something concrete (or at least much more concrete) I can base my reasoning on.
The CS argument does have precedent -
see the list here. I'd say it's 99% reliable as an indicator.
Every time a CS has been added, it was when a DLC or Gods and Kings introduced a city state that would have replaced it. In Gods and Kings things got a little wonky because of the inclusion of two new CS-types, but even then all moved or removed CS were properly replaced; since BNW doesn't seem to add any new CS types, it's safe to assume then that Sofia
is a replacement for something, just like how Ur and Riga, being maritime CS, are likely replacements for Rio-Brazil and Lisbon-Portugal, which are both currently Maritime CS. As for Belgrade, Belgrade was originally considered as one of Sofia,
not Riga's, possible replacements (since it is also a militaristic CS), and its elimination only increases the chances of the other militaristic CS getting an update.
Lastly, I do agree that Vietnam's archetype as a civ will not rest on ideology. If it is in, I believe it would probably have to do with turtling or some sort of defense. Then again, remember that not all the civs have to have UA's the make 100% sense - look at how China gets stuck with a military UA that would've fit Germany or Mongolia even though China has tons of achievements elsewhere; or how Greece gets a city-state UA when it could have gotten a science or trade one.
Isn't speculation fun?