Here's a quick list of my favorite MashMorgan fics :3
Humor
Guda/Morgan and Mash/Morgan. In which Morgan has recovered SOME emotional range, but "shame" is not in it. Sends me in stitches anytime I read it.
Double Trouble Boil and Bubble.
In which Tonelico and Morgan are fighting for Mash's affection. Absolutely fucking HILARIOUS to see them both getting super competitive while also blatantly thinking that the other is cringe.
Mash being Not Fucking Normal about Morgan
In which Morgan accidentally hurts Mash while kissing her. They both have opinions about that.
Three Phalanges and Two Metacarpals.
In which Mash uses a catalyst to summon Morgan.
What happens to Morgan after she dies.
Morgan-centric canon retellings
In which Tonelico goes Through It. Heartwrenching.
A longfic on Tonelico's journey to become Morgan.
Other
nsfw, Mash and Morgan have weird inhuman sex.