Fractal is not multiplatform (i.e. isn't available on Android and on iOS) and Matrix can be confusing to people not already familiar with it.
And no, a wall of text explaining what Matrix is won't help since most of Discord's users are teenagers with a very short attention span that don't read much (unless they're forced to by school).
Potential solution, may be controversial.
Just add a vertical video with Minecraft parkour or with CS surf on the bottom and a half naked woman from a freelance platform explaining what it is.
There are also Element and SchildiChat as alternative clients.