I think RDA would've beat him pretty convincingly. His style would be very tough for Frankie to deal with. He has all the skills to beat Frankie but his biggest advantage is his power/overall strength. He would outmuscle Frankie and would land the more significant shots on the feet and take the decision, but I actually wouldn't count out a KO/TKO for RDA either. Frankie has a great chin and amazing recovery, but so does Bendo and he got KTFO by RDA pretty damn easily, those shots would've put down pretty much anybody, I think Frankie wouldn't get up after that.
Out of everybody at LW, RDA is that one guy that I think would truly beat Edgar. He might even beat him more dominantly than Aldo did in the rematch because of his killer instinct. RDA is a bad fight for Frankie IMO. Can Frankie win? sure, but it'd be very, very close - if RDA were to win, it likely wouldn't be that close.. at all. RDA is a fucking monster, too skilled and powerful for little Frankie. The LW division is a whole different world now compared to what it was when Frankie was on top.
I can see Frankie definitely beating Pettis convincingly, but I think Melendez would be a close one. Frankie/Giblert, to me, is one of those fights that would be razor close and no matter who gets the decision, it would make sense simply because it was that close. Yeah, I may be wrong but stylistically, Giblert/Frankie match up extremely well and I think Giblert would give Frankie a very tough fight, I think it'd be one of Frankie's toughest fights to take on at LW if he were to return there. I expect many to disagree, but it's whatever, just my opinion, homies.
He'd never fight Alvarez though
Which sucks because Edgar/Alvarez would be such a great fight.
I could see a fight between those two being like an instant classic, some GOAT level war shit.
Now I'm a little upset because we'll never see it (for obvious reasons) just like you said. Shit.. I never thought about this fight before until you mentioned it here - the way they match up with their styles and such, that's like some dream fight shit for me now, just like prime Thiago Alves vs. Paul Daley during his UFC run.