Criticisms of Atlanta trap power trio Migos often focus on the group’s perceived lack of flexibility, or their reliance on a particular musical gimmick. After the success of their 2013 mixtape Y.R.N. and the Drake-bolstered “Versace,” Migos made subdividing Gucci Mane-style flows into distinctive configurations of three not just their signature gesture, but a veritable — and sporadically irritating — trend in hip-hop. However, as their newest mixtape, Rich N***a Timeline, makes abundantly clear, Quavo, Offset and Takeoff’s rapping has gestated fairly dramatically over the course of the past year and a half. On the sprawling six-and-a-half-minute opener “Cross the Country,” they announce themselves with deft, speed-ramping cadences that are wildly advanced from their initial, more one-noted fast-flow experiments — full of inventive, sputtering micro-rhythms and melodic interjections.