the Tang emperors held Turkic and Mongol titles.
And that makes perfect sense to old Imperial geographers.
Saying the title would be "meaningless" is ignorant on my behalf, I should've said in most circumstances it would probably be
already assumed, because to the Emperors, China was the center of the world. Literally, it was the center of civilization, and the Emperor ruled 天下. That is, all things (big or small) under heaven. So Rome is, under old Chinese imperial thought, probably already considered to be a vassal state to China. Caesar would just fall under the Emperor's other long list of minor titles all subservient to the 皇帝 of 天下.
At the same time, I'm not a savant when it comes to Romano-Chinese relations, and in some classes I've taken on Asian history during the Qin Dynasty there was some pretty interesting stuff from the Chinese on the Far West. They called it 大秦 (Great Qin) and made some interesting comparisons between it and China. If Rome manages to conquer the Sassanids and Persia and gains a link with China, I'm sure a war breaking out between them over Central Asia and the Romans losing, etc. could lead to an Emperor claiming the title alongside his many other ones- and perhaps they'd look at it as a sort of "great second" to China. It's interesting to note though that the Chinese thought Antioch was the capital of Rome, and the sources that I read seemed to only go as far west as the Levant (particularly Syria). But claiming titles was less of a big deal for most native Dynasties in general, as they were all sort of just assumed under the big bubble of the Emperor's title.
If not by marriage and (long-lasting) conquest of Persia, if the Romans (or Greeks) found (or maintain) a presence in Bactria/Central Asia and grow a larger sustainable, it's entirely plausible that if the Emperor conquered them he'd pick up the title if the locals used it (and I imagine they would if Bactria got split off from Rome again).
TimelineExplorer said:
What is the Chinese translation of Caesar anyway?
Phonetically it's probably 凯撒(kǎi-sā). I imagine 凯撒is what they'd use, since even in Rome the term was sort of just passed down from Octavian onwards- it just relates back to Caesar himself and doesn't mean anything etymologically in and of itself (but certainly it carries a lot of semantic weight). There's nothing to translate in that sense, just what it sounds like.