Would it be to hard for Bethesda to release a super hi res 15th anniversary version of the game? Kinda like what they are doing with the Halo Anniversary...

Yes, it would.

There are rumors the source code was lost when they moved to their new office years ago, and even if that rumor isn't true, the source code is a horrible mess of assembler code (they changed lead programmer 4 times during the development) that would take a long time to turn into a good modern clean code.

I don't pay any attention to anything Halo-related, so please bear with me, but what do you mean by "super hi res"... a remake? Regardless, anything involving tampering with the game's files or remaking anything is highly unlikely.

You can always wait for DaggerXL.