OctoPi already includes a cura plugin and cura binary. What about adding a slic3r binary? Do you think that some people would want it? Slic3r is actively developed and very customizable. Some people say that it's better than cura, though it's slower.

I've worked on the install instructions for RPi. if I understand correctly, to add Slic3r, the build would need to be part of this?

it would make the image larger, however, and OctoPrint-Slic3r isn't bundled with OctoPrint by default so maybe it wouldn't be used much?

Anyway, something to consider. If you want Slic3r as part of OctoPi, let me know and I'll try to help.