I'd love to see a giant, towering mech, just a little bit smaller than your average height sized map, it would move extremely slowly but have massive armour, have devastatingly powerful weapons and fists that could punch through whole concrete segments with ease, can someone make this? The coalition punisher mod just isn't quite big enough :/

Try this mod.
GODDAMN Corp

Goddamn or the Punisher/Behemoth are really as good as you're gonna get. Large units get harder to program and glitchier the bigger you make them. Just try the Zaku from Goddamn to see what I mean.

The goddamn is the right size, but it's not exactly pretty and well weaponised, could someone like modify it to be absolutely devastating? With missiles, fists, etc

And he is rather buggy :/ His legs like to flail everywhere

There's also this one from Naxete. Hopefully it's destructive enough for you:
viewtopic.php?f=61&t=29687

SWEET, Liberty prime is just epic