The ability to increment and decrement the file pointer..
ex.:

/finc name [n]
/dec name [n]

finc bleh ; increment the file pointer by 1byte
finc bleh 10 ; increment the file pointer by 10bytes

fdec bleh ; decrement the file pointer by 1byte
fdec bleh 10 ; decrement the file pointer by 10bytes

I know it could be scripted like that

alias finc {
var %n = $2
if (!%n) %n = 1
fseek $1 $calc($fopen($1).pos + %n)
}

but it would be faster if it was built-in mirc