On the Southeast Asian front, for 1178 Jayavarman VII is an obvious bookmark - he's just a few years away from ascending the throne of the Khmer Empire, though he'll need to fend off a Cham invasion along the way. Jayavarman VII is basically the standout personality of the Khmer empire and the only one for whom we have peronalized portraiture. He was a Mahayana Buddhist coming to the throne of a Hindu kingdom, he had big ideas about how things should be run, and despite being about 50 when he took power he was able to rule for four decades.
Other options aren't quite as filled out. King Jaya Indravarman IV of Champa, who launched the invasion of Angkor, could be a good foil, but I'd say it'd be more daring to do Vidyanandana, a Cham prince who allied with Jayavarman VII to counterattack and overthrow Jaya Indravarman IV and eventually take the throne of Champa himself. He could start out as a landless adventurer perhaps, which would be an interesting twist.
I'd also recommend a bookmark for an early Thai/Tai ruler. This one is a bit tricky because while they were definitely on the scene prior to 1178 and occasionally appear as lords of various cities, the founding generation of sovereign Tai rulers don't take the scene until after Jayavarman VII's death 4 decades after the start date. We also have names for the fathers of that generation, but even they may have been young/not born yet in 1178. But even without a prominent name to use, I do think playing as an upstart Tai ruler rising to power in the shadow of a declining Angkor is a fun setting and would showcase the mandala mechanics.
For the earlier strart dates in Southeast Asia, I think we're on much less solid ground. We don't have many personalities that loom large in the rather limited epigraphic evidence we're limited to. There's Airlangga in Bali and Java, but he unfortunately died in 1049. There are certainly names of kings to fill out the holdings, but I'd be at a loss filling out a bookmark.