I am just gonna toss my thoughts out on what the abities might be for Echo and what I would like this character to do.
My first speculative point will be:
This one is a cliche and probably predicted by most people so onto my next point.
Second speculative point will be:
•Shield based abilities
This one is probably also lacking in the originality department, but with Symmetra losing all her shielding abilities directly effecting her teammates health, Echo now has the potential to fill that role that has been stripped from Symm. I personally would love to see a shield based healer.
Third speculative point:
This point here is a little more creative but I would like to see her primary fire be a sort of volly slow moving heat seeking chromatic spheres that make a wailing sound and seek out targets but can be dodged. On impact they have a chance to cause brief fire damage, frost slowness, or some other third effect.
My final speculative point is:
Just like her namesake echo will effectively be what its called. The ability to copy herself or teammates as hardlight, being sort of like mirror image from world of warcraft. Just as the echo ability in hearthstone allows you to make a copy of your played card until you run out of mana, echo will allow her to make echos of herself or teammates as a sort of ai clones that surround her for a period of time, all being produced from hardlight. Each moving and jumping with her as if controling multiple heroes at once.
That concludes my random Echo speculation list.