This could probably go in either pre-1900 or post-1900, but I'll put it here. The question here is the effects happen if France grabbed Thailand's
Isan ("Northeast") region during the period of colonisation in Southeast Asia.
The Isan region is populated mainly by people who spoke dialects of Lao--in fact, speakers of Lao in Thailand outnumber speakers of Lao in Laos. The region was dominated by the Kingdom of Lan Xang, the ancestor of later Lao states. It was conquered by Thailand in the early 19th century, but later Thailand would lose most of this area east of the Mekong to French Indochina.
One of the reasons France didn't push much beyond the Mekong OTL is because the British preferred to keep Thailand as a buffer state, and it wouldn't be good if the French grabbed too much of it. France did grab a bit more of Thailand in 1904 and 1907 though. How might France be able to grab more of Thailand? The Entente Cordiale kept either party from making any substantial annexations of Thai territory, but it also dealt with a lot of other colonial matters. Could it have gone differently to allow France more Thai territory (the Khorat Plateau which makes up the majority of the Isan) in exchange for concessions to Britain elsewhere in Asia or Africa? Britain themselves could gain more of Thailand in exchange for this French move, by annexing Chiang Mai (only abolished in 1899) or perhaps some more of southern Thailand (although perhaps its easier for this scenario to assume Thailand only loses the Khorat Plateau). We'll assume the Great War happens as IOTL (and ends similarly), so further colonial expansion in the region doesn't happen and Thailand remains independent.
After decolonisation (we'll assume WWII goes similarly as well), Laos will likely gain all the land in the Khorat Plateau/Isan. Would it still become a communist state? It's probable that Laos will still have a major communist movement, and that the Laotian Civil War could still happen. Since Laos will be a much larger country with a much higher population, this could make the Second Indochina War all the more chaotic and cause greater US involvement in the region and especially will cause greater Thai involvement. Laos being partitioned into pro-communist and anti-communist portions along the Mekong River would be pretty interesting.
Would Laos be more developed than OTL? With this area, it would likely have a population of about 30 million, many of whom would migrate to the capital city (as they did with Bangkok) which would still likely be Vientiane. Vientiane would be several times larger than OTL and a much more important city globally (Bangkok would be smaller, but still a very important city). It would be the fifth-largest landlocked country in the world in terms of population, which would continue to make development challenging.