Apart from the obvious reason is it worth using such a password encryption method.
I can see the obvious point. I did consider using a AES method myself but then thought well actually. Is my game going to attract that *much* attention. Also is a hashed salted sha512 suitable for my requirements. Yes it is quite easily.
Just think its overkill for something this level but I suppose if the engine is going to go through a major overhaul then it doesnt hurt to just implement it.
As for mysql,mysqli,pdo
PDO is going to reduce the amount of users that can install and run the engine. Its not installed out of the box when compiling php.
Mysqli preferred over mysql.
Recaptcha. Hate it. I do use it. But cant stand it. Its at 99% of the time hard to read. Not fool proof anyway.
Smarty. There is better ones out there if your going to go down such a redevelopment route anyway.