Put me in the "unconvinced" camp on whether BG are too good.
NetEA lists :- While using an SR5 and Init +1 the costs have been increased and the armoured units have been removed (Wraithguard and WraithLord). "Spamming" these with Wave Serpents means that at most you can get 7x such formations with possibly an unmounted BG or some rangers 8-9x formations. If some ratio with Guardians is implemented that would drop the BG to ~4x formations which does not of itself seem 'game-breaking'.
Adding teleport is an intruiging idea that would potentially mimic the Elysians to some extent; teleporting essentially unarmoured troops near to the enemy. The point here is that, while the BG are indeed likely to initiate an assault, being unarmoured without inspiring characters are significant drawbacks.
Up to now, deploying the Avatar has always been considered simultaneously with other teleporters. Bringing the Avatar on with some BG formation(s) is the equivalent of BL teleporting a demon prince in with their terminators, but with significantly less formation resilience; if the Demon Prince is not considered a "problem" (except by me

) I do not see why this should be a problem.