well ... imho, it is not salvagable.
a rewrite is not a salvage - it is something new ... the only thing that is salvagable, is the core of the game - its foundation. but the story (what is most important) is a trainwreck.
it is full of dropped plots, cheap tearjerkers, lore-breakers, plot holes, cheap writing tools and character assassinations.
- cerberus mutated from "underground organisation" to "military force" (from where did they get all that eezo to build all those shiny ships?)
- geth are suddenly a platform based ai? wtf lore? mass effect and mass effect 2 made it clear how geth work.
- dark energy ... the mysterious force that ages stars (buildup for either being nemesis or salvation) ... dropped.
- shepards mental state or better decline is awful .. shepard was meant to be a brick (to allow rp) and it worked for 2 games - never change a running system.
- thessia - no comment
- crucible ... seriously who let the blizzard guys in? superweapons are fun in blizzard games because you somehow expect it but in a mass effect game?
- london and the battle for earth ... the lore itself is crying and sniveling. after the first salvo, earth is a wasteland.
- synthesis ... while dealing with the possibility of a technical singularity is a popular theme in scifi, it was never a problem in mass effect, because it already happened billions of years ago (reapers). it was not even man vs. machine (geth are not really the bad guys) - it was a classic good vs. evil concept, that was wrecked with the starchilds "explanations" and the writers blunt attempt to blurr the lines between good and evil.
- garrus ... leaving ... shepard ... behind ... wft?!? i guess "i am with you shepard", "right behind you" ... were just empty phrases.
i could go on and on ...
basically, many things went wrong with the plot but the ending/stachild sequence jumped the shark. mehem is a "digital duct tape" that prevents the story to turn from "awful & lore crushing" to "this is forbidden by the convention of geneva".
if you really want to rewrite mass effect 3, cool ... do it. but you are going to need a new voice over, level design, a (real) writer and an editor who is worth his/her salt.
good luck and have fun .... and please create a fresh topic / group.