There haven't been any announcements of new Mega Pokémon being added to Pokémon GO since Septembered three Mega Events, which unlocked Mega Pidgeotto and Houndoom in raids, along with securing a late-October research rewarding Mega Gengar Energy for those who completed September's research. There has been data found in Pokémon GO's code, though, that reveals two new Megas that will be coming to the game soon: Aerodactyl and Slowbrow.
The PokeMiners of Silph Road have discovered in Niantic's latest update new 3D models for Mega Aerodactyl and Mega Slowbro. This has not yet been announced, but previous datamined finds include Mega Energy itself, Shadow Mewtwo during GO Fest, Magikarp Community Day, and a laundry list of other features that have since been added to the game. It is a good bet that these two will be arriving in the game shortly, likely after Mega Gengar makes its Research debut during the Halloween 2020 event.
All there is left to do with these two is speculate. Aerodactyl is likely to be the more popular of these two, so I could see this becoming the next Mega Raid to be added to the rotation. With its killer design and high usability, Mega Aerodactyle is a raid that Pokémon GO trainers will want to get after. Slowbro, on the other hand, may receive the same treatment that Beedrill did in September, with Timed Research and Field Research rewarding Mega Energy.
Also, if Slowbro does actually join the Mega Raid rotation, one thing of note, is that Slowbro, and indeed none of the Slowpoke evolutionary line, has not been made available Shiny. One of the allures of Mega Raids is the possibility to encounter Shiny Pokémon in their evolved forms. If a Mega Slowbro raid were to coincide with a simultaneous Shiny release of Slowpoke, that is something that would active the Pokémon GO base. As news comes in, Bleeding Cool will report.