Most of my stories actually play in the same world. So while I don't have crossovers yet, they'll eventually come and yet there are references. Though, as it's its own named fantasy world there's not much of a chance for crossovers with other franchises to happen. On the other hand, I already gave Saril permission to use my world and am generally open.
Though, who's basically cut out for this kind of event would be Iori from my novel "Eldritch".
My adolescent eldritch goddess whose panic attacks when things again get too crazy for her are probably the greatest threat to the multiverse (it's a complicated dynamic). As she's capable of dimension-hopping she could basically show up wherever. Personally, I even like to imagine her showing up in different franchises.
Her future versions are going to be a lot more aloft in mentality, more playful, and willing to play along. From Chapter 153 -155 there's an appearance of a version of future Iori, yet this one is basically the absolute end. The normal future Iori would rather be somewhere between her angsty current self and the omega version that showed up.
Iori's general thing is that she's a ridiculously powerful being that can manifest everywhere, growing every kind of form or horror, but has very human sensibilities.
So if anyone is interested and would handle her in a sensible manner, I wouldn't mind having her being used. You may send me a message.