I agreed with brackets idea. lvl 10-40 and 40-50 in my opinion would fit best. Gear on these lvls makes big difference against lower lvl players, especially lvl 50 gear.
About premades, i am against it, because, as someone alerady said, 4 team premades offen prove being devastating against randoms(altho i have seen premades being stomped by pugs) Maybe some special mode of warzones would be fit for players who desire some real team action?