1) Hash check on d3d9.dll and other critical game files.
Most cheats seem to use d3d9 to tap into the game.
By making a hash check before launching the game, and halting if the hash is invalid, most cheating nubs could be prevented from loading up the game and pestering good honest players.
(And good honest server owners/admins. )
Hell, this hash could be sent to each server and verified there, to prevent client-side hacking.
2) Cancel-able connect event?
Could be useful if you want to deny access without using the standard ban - if only to give a custom ban message etc.
Bascially, by cancelling the connect event one could prevent a player from joining the game in the same way a ban does.
3) Extended client-side file IO
Very useful indeed, if it turns out the serial system gets cracked. (Like it probably will. Survival chances are slim, let's face it. People who could've used their skills to do something constructive like bringing peace to the world, are instead making sure 11yo moronic kids can cheat at online games. Jesus.)
Would have create-read-write access to for example the whole C:\ drive, but only to special files - to prevent people phucking up people's PCs using MTA. For example, files created with mta could have a little tag in them, etc.
The point is not to access the client's files or change them, but to be able to save game data in places that make them harder to find, access, tamper with or delete.