This is a dream come true! Awesome.
By all Fifa 21 faces do you mean the new ones or literally ALL? Complete face pack would be wonderful.
I even see Alex Morgan - female faces/players for Fifa 14 is something I've been really interested in.
18, 19 and 20 faces are also great news, because there are players who aren't present in FIFA21 default database (retired/moved to obscure leagues) e.g. Sydney Leroux but had their face scans and could be used in extended databases with mods.
UPD: Also looking at the 21 Neymar face (bald & bearded), have you checked if his head model has changed? Would be cool if the head model stayed the same and we could also use the hair from your 20 Neymar test conversion, assigning both on radom with Revolution Mod and having a bit of variety in games, you never know which hairstyle will Neymar have this time.
Also, the same (latest) head model could be used in combination with different hair models that would fit it seamlessly converted from previous Fifa titles for Classic projects
Players like C. Ronaldo, Messi, Neymar, Pogba, Griezmann, Sergio Ramos could benefit from having a 'standard' head model and different styles of hairstyles to choose from/to assign for randomization: