Can a successful applicant in the New gTLD Program: 2026 Round choose to sign onto a different form of the registry agreement other than 2026 Base RA?

No; all successful applicants are required to sign onto the form 2026 Base RA.

Note if an existing registry operator applies for allocatable variant strings of an existing gTLD in the New gTLD Program: 2026 Round and is approved, it must transition to the 2026 Base RA.