Honestly, having Ellie get Angel’s power, instead of Tannis, would have made a lot of sense.
Which, of the two of them, is better with machines? Ellie. Can you imagine a mechanic with the ability to control machines? She could build awesome warbots and pilot them into combat. She could amass an army of vehicles and drive them at enemies herself, via mental remote control.
And hell yes, she would be good as a siren.
Doing it now, though, would be difficult. There’s only 6 around (no idea about the 7th which got hinted at), and most are accounted for. Lilith has her powers back, Ava has Maya’s, Amara has hers, and Tannis has Angel’s. Tyreen’s power probably transferred to someone new already (since her body vanished) and Ellie didn’t gain tattoos (or maybe Tyreen’s power was destroyed by her combining with the destroyer. Who knows?).
That only leaves one set of powers remaining, which are presumably the ones that Commandant Steele had. We discussed this a while back here: New siren type - Phasewake - while talking about what the sixth power could be.
Since we never see her use them, we don’t know what her power was, so it could be something that would fit Ellie, like huge phase-explosions.
I just don’t want to see it be “Ellie is fat, so she gets fat powers”, like gravity powers or something. Like OP says, “bl3 is woke az hell”, so they should make a plus-sized siren (Ellie or someone else) with powers like anyone else, not make it a joke about her weight.