FlutterDash is like… this weird guilty ship for me. I’m pretty convinced that a relationship between them would be emotionally abusive and unhealthy as all get-out, because while Rainbow Dash is capable of being a decent friend to Fluttershy most of the time, them being girlfriends would bring out the worst in RD. She’s already inclined to be impatient and has been shown to have a hard time really understanding Fluttershy’s super-gentle, pushover demeanor.
 
And Fluttershy not talking back and just folding when RD pushes her around would spiral into worse behavior because RD would subconsciously start trying to goad Fluttershy into biting back - it would essentially become a competition for her and she’d get frustrated with Fluttershy for not even trying to join in.
 
Plus, I could see Rainbow getting super possessively jealous of Angel in a hurry.
 
Fluttershy of course would just walk on eggshells and accept any and all abuse, which wouldn’t be good for her or RD. Then when it goes too far and it pushes Fluttershy to tears, Rainbow would be genuinely sorry and apologize, but she’d still end up doing it again sooner or later because Fluttershy is afraid of her own shadow and seriously Fluttershy?
 
So I’d totally read the hell out of fic that explores this absolutely terrible, unhealthy trainwreck of a relationship (including any attempts at intervention from their friends) in a good way. But I have absolutely no illusions of it being a healthy relationship. Honestly of the main characters I think Rainbow Dash is the least suitable partner for Fluttershy.