Bao Dai is smuggled out of the country towards the end of the Japanese occupation so that he isn't pressured by them into declaring independence from France with that being done by Cuong De instead. One his return he's in a slightly stronger position so isn't talked into abdicating by Ho, which in itself could have some very interesting effects. He attempts to walk a path between the status quo and full independence lobbying France for increased autonomy, but when he sees which way things a going publicly declares for an independent Vietnam. The French who are not at all happy with this effectively depose him and he's sent into exile, boosting his nationalistic credentials. After independence he returns to South Vietnam as Emperor and Head of State in a ceremonial monarchy, his increased popularity over our timeline means that Diem is unable to oust him but instead sees him preside over a string of changing Prime Ministers.
Alternatively Duy Tan doesn't die in a plane crash in 1945 on his way back to Vietnam and replaces Bao Dai as Emperor as part of Franch trying to combat the Viet Minh. This however backfires as over time he takes an increasingly independent path which eventually sees him removed and sent into exile again. When Vietnam achieves independence he returns as Emperor, at least in the South. In both these scenarios you still need to either see the Viet Minh headed off or at least South Vietnam somehow survive - one idea for that might be no Watergate as without those distractions I can see Nixon bringing the US Air Force hammer down on any attempted invasions by the North.
