I agree broadly with Skallagrim. The most likely outcome of any 'Muslim conquest of Francia' after Tours (732) is the Muslims taking a few footholds in the south (Septimania, Toulouse, the Rhone Valley) and holding them for ~25 years at the most before the Great Berber Revolt and the Third Fitna choke off their manpower and they collapse/are driven back into Spain (or possibly further, depending on how well the Franks and any Christian remnants in Spain are situated at the time). There might be a few holdouts a la
Fraxinetum, raiding bases hidden in the marshes of the Rhone or the
calanques of Provence. As a side note, I've been interested in some sort of scenario where the Arab settlement at Fraxinetum-Sant Tropez expands into something permanent. For later times, I suppose.
As for expansion, I think the two easiest options are East Africa and Indochina. All the former needs is to continue OTL trends of Muslim expansion into the center of the continent in the 18th and 19th centuries, though what drives this I'm not entirely sure. Indochina seems a lot more interesting to me. If we push back (or entirely avert) European colonization, we have a powerful, populous, Muslim Bengal just over the mountains to the west, looking to expand its influence eastward like it did IOTL in Arakan. Historically, Islam spread well along trade routes, so the trading centers of Indochina seem like natural next-steps: if I had to guess why it didn't happen IOTL, the two big ones seem to be a) 'no Muslim ruling class' so there was no real cultural drive and b) the presence of Buddhist monasteries as a center of social life, creating an incentive for the lower classes not to convert and to keep the faith, limiting any inroads amongst the poor and limiting Islam to the mercantile class.
Off of five minutes of googling, a workable PoD might be keeping Dhammazedi from ascending to the throne of Hanthawaddy in the 1460s/1470s, or preventing the rise of First Taungoo in the early 16th century. Before Dhammazedi, the religious state of Hanthawaddy was a chaotic mess, many temples/monasteries practicing their own rituals and neglecting social duties; he imported Sri Lankan clerics and forced a universal reform of the monasteries that destroyed the worst of the corruption and created a standardized religious system Intertwined with daily life. The Taungoo adopted these reforms and imposed them on monasteries across their empire (most of Burma, Thailand and Laos).
If we prevent these reforms, the situation in the 15th-17th centuries is one ripe for conversion. The temples/monasteries/pagodas are blatantly corrupt and neglect the poor, syncretism and local sects (including human sacrifice) abound, and there's no real religious orthodoxy to be defended by educated clerics. Also, without the Taungoo, there are a number of rival kingdoms and petty statelets, none of whom can present a unified opposition to the spread of a new religion. We get the typical 'scissor pressure' (my term, if not a very good one) of Islam spreading syncretically among an oppressed lower class by offering social support and justice in the hereafter and among the upper class to further ties with the Malays and/or Bengalis and to pick up some of their prestige. I imagine that there are a few generations of syncretism and wary coexistence between Islam and Buddhism before the Muslims gain enough of an institutional* or popular advantage to launch a 'renewal' movement that expels most pagan beliefs and consolidates power.
The biggest roadblock in the scenario would probably be some kind of 'later revival' of Buddhism in the hill country or Mekong Plain, probably on the back of some developing power, that can credibly challenge the rise of Islam, or more simply Vietnam. IOTL, the Vietnamese were one of the more powerful Indochinese states and never cared for Islam (or Christianity, for that matter), seeing it as a foreign faith and a threat to the state ideology of Vietnamese Confucianism--though this wasn't helped by their main interaction with Muslims coming through Champa, their ancestral rival. I could see Vietnam turning into a bastion of Buddhist orthodoxy and anti-Islam sentiment, probably intervening in the west and south to make sure their coreligionists hold out. However, given good relations between Muslims and the Ming government, I doubt the Chinese would back them--unless some sort of Qing-analog arises, in which case the loyalty of Muslims to the Ming could create some sort of 'stop them there so they don't come here' mentality. Regardless, I think Islam would have a hard time spreading in the hill country, and any Muslim states in the lowlands would struggle to control their borders with the highlands.
*Now that I've thought of it, the weakness of Buddhism might create an institutional 'need' for Muslim officials; with no temple system to help run tax collection in the decentralized
mandala political system, state will have an increasing dependence on trade revenues; as a significant number of involved traders would be Muslims, this would increase their political and cultural influence. Might not amount to anything, but still.
it takes is Champa beating and rolling back the Dai Viet.