You are free to implement any filesystem driver, and either load it in your custom implementation of EFI, or from an EFI image stored on a FAT partition. The EfiFs project provides many drivers under a GPL, including exFAT, Btrfs, ext2-4, NTFS, ISO9660 (cdfs), ZFS and also HFS(+).
But who else can use that?
You are free to implement any filesystem driver, and either load it in your custom implementation of EFI, or from an EFI image stored on a FAT partition. The EfiFs project provides many drivers under a GPL, including exFAT, Btrfs, ext2-4, NTFS, ISO9660 (cdfs), ZFS and also HFS(+).