Tau would require too many various character models to implement, as aside from Fire Warriors of various armaments their army consists entirely out of mechs, drones and mercenaries. Dark Eldar, and basically any other "mercenary race" would be better used as RTC Elites than as a standalone faction, especially given the DE gendered society, breaking the lore on which probably wouldn't sit well with GamesWorkshop(even though technically the "Banshee clause" applies to all of them as well). Necrons are the most likely to happen from a technical standpoint(lowest model variety, each class is basically a progressively more ornately decorated version of the Necron Warrior), but they would require bE to embrace asymmetric balance at a whole new level and/or a significant amount of power level shifting to accommodate troops of various ranks being all used as base classes(comparable to the Painboy).
http://www.belloflostsouls.net/2015/10/40k-new-tau-fire-warriors-weapon-rules.html http://www.boyzbenerds.org/40k/ig/download/imperial-guard-reference-sheet.pdf so a tau can not compete with a space marine or a chaos marine and the other races, if we consider a 1 vs 1 fight
