I think they should give all rogues an Ambush ability that can only be used in the surprise round and does oodles of damage, then scale down the damage of Sneak Attack just a tad. That way, they can let thugs gain advantage with just one flanking partner, a la 3e.
Then, they could give Thief-rogues advantage against a target who attacked someone else (and not the rogue) on their last turn. That sort of models backstabbing (attacking when the enemy's attention is diverted) without actually having to introduce facing or more positional stuff into the game.
All this DOES add up to an awful lot of attacks with advantage for the rogue, but hey, maybe that makes up for the slightly lower BAB than the fighter?