It's cliché but what do you (does everyone) think of a Sue Richards metamorphosis for Black Widow.
Natasha would remain a crack assassin but be able to turn invisible and either skate around on invisible waves or shoot invisible rays or both. The invisible rays are my choice to stay. The rays would be absorbable and not cause too much damage. Then, there's the force field. It won't be on Invisible Woman's scale, but it can stop a bullet or a medium energy bolt. Input?
She will be who she's always been. I'm going to look up several histories to use as source material for her history. And, as far as her personality goes I think I'd define her as optimistic, realistic, fast, and stable and well enough to cover her own ass.
You could just play as Sue and give her a little martial arts training. For a costume, you could put her in the Future Foundation (white
) suit or perhaps the one from the 2015 movie
I'd also suggest going with maybe Jean Grey or Rachel Summers. They each have both telepathy and telekinesis. They can use their telepathy to make themselves appear invisible to others and can use telekinesis to generate force fields and force bolts. You'd probably have to work with the GM though as Jean and Rachel are extremely powerful characters in the comics.
If you have your heart set on Black Widow, then might I suggest the following:I don't know if @CaptainManBeard is going to let you start out with assassin training since all of our characters are supposed to be teenagers. But, you could give her some training in a martial art or two. Maybe add talent for pickpocketing or lockpicking too.
Instead of outright invisibility, you could go the Miles Morales or the Predator route and give her a camouflage ability. Natasha, along with her clothing, could blend into her surroundings. It's basically invisibility, but it requires you to be near an object such as a wall, a tree, some brush, etc. It also keeps that arachnid theme going too because spiders can't turn invisible, but they can blend into their environment.
Second, you could give her the "Widow's Bite" she has used in the comics. I would have it be an actual power, instead of being shot from gauntlets, it is shot from her hands. The Widow's Bite is a short-ranged energy bolt used to stun and knock out opponents.
If you wanted to keep the poisonous spider theme going, then you could instead give her a Poison Kiss. This kiss would make victims incredibly sick and nauseous for an extended length of time. You could also give her a high resistance to toxins.
EDIT - Also, in the comics, Natasha is an enhanced human much like Captain America but on a lesser scale. She can lift 500 lbs and run up to 36mph. She has peak human durability as well as the agility, speed, and stamina of an Olympic athlete.