While not an official replacement, Mears Connect will be a third party offerings to effectively replace the void left by Disney's Magical Express:

Quote Originally Posted by Roger Chapin, Mears Transportation Spokesperson

The Mears Connect service will initially link visitors arriving at MCO to resorts in the Walt Disney World area aboard “high occupancy vehicles” and offer similar amenities — including luggage handling service, return trips and convenient staging at the airport terminal, enjoyed by passengers on Disney’s Magical Express since 2005.

It is important for visitors to know they still have many of the same safe transportation options for their Orlando visit, all offered by the same company who has been providing seamless, economical, and hassle-free transportation for decades.
Guests can begin to make Mears Connect reservations for 2022 trips in May 2021. Pricing for the new service has not been announced at this time.