• Surviving Elamite/Khuzi. The geographer Istahri wrote in the 10th Century of an unaffiliated language spoken in Khuzestan (ancient Elam). I'll assume this is a survival of some form of Elamite. The Khuzis adopt Islam, but a different branch from the rest of Iran (possibly Sunni or maybe one of the Kharijite sects)and maintain their language through the 'shield of faith'. They are further encouraged by nationalist movements in the 19th century.
• I always thought a surviving Kassite language in the Zagros would be cool. A few isolated mountain communities or nomads may preserve the language, although it would be flooded with Iranian and Arabic loanwords. Actually the Zagros as an Alternate Caucasus with a greater variety of languages would work well since this sort of happened in OTL.
• Nahuatl survives as a national language of Mexico. (There are still Nahuatl dialects spoken in Mexico today, but without any kind of official support). The royal Cédula of 1770 is never signed and things continue like they had before, with Spanish in the cities and government, but Nahuatl remaining the lingua general.
• an idea I've thought of for surviving Egyptian language is a class of swineherds (pigs were considered an animal of the god Seth, and unclean) were somewhat segregated in ancient Egyptian society, and they eventually form a caste unto themselves and even after Christianization and Islamization they remain separate, speaking their own Egyptian language even after everyone forgets exactly why.