Imo Law should've been. He could have both Punk Hazard and Dressrosa, if only he was the one defeating Doflamingo (with a mid-fight PU of Ope2 Awakening), while Luffy matched Fujitora in a serious battle.
Kidd should've also beaten BM in WCI while Luffy being busy creating powerups as preparation to beat Kaidou, and Zoro rescuing Sanji and giving him a loyalty lesson. So imo Kidd (again with mid-fight PU against BM) should've been an Emperor with WCI as his territory.
While Luffy can have Wano after beating Kaidou. While Blackbeard can kill both Shanks and Dragon or something. So still Four Emperors.
Well it's just my alternative plot, strength-wise they're both still lacking quite a bit tho.