Her charitable work is through her own foundation, which supports young people's lives through athletics, particularly those from disadvantaged backgrounds.
I don't know if she would be interested in taking on more.
And even if she did, Harry is still the Founder and I don't think he can be got rid of.
He wasn't "given" a patronage. He wanted to bring the Warrior Games concept in to a different and international arena. It was incubated via the Royal Foundation as that was his charitable vehicle at the time, and when that slit his project went with him. Of course, being Royal opened lots of doors to get the Games off the ground - good staff and MoD support being key elements of that.