Casting Spellword is:

Imaginarium Mentalus

I assume the cast we can see through his/her own illusions.

Used this to create an image of a medusa. The party was so afraid to look at it that it took like 15 minutes, real time, for them to realize it was an illusion. One of the most entertaining DM decisions I have ever made. 

 So i'm in an evil campaign right now and I have a half-orc wizard. I plan on using this to fire an illusion of Firebolt at like a bar owner or something like that and have my wizard say "If you don't listen to Grok, Grok will hit you with that again, but real"

Interesting that it doesn't require Concentration. I was expecting it to, really opens up options. 

Could we create a sonic illusion that appears solid to echolocation? ie. Swarms of bats are waking up on the cavern ceiling. You duck down and create an illusion of a boulder, but meant to be perceived as stone to echolocation, not to fool eyesight.

Forest Gnomes get the cantrip as well.

As a forest gnome creating an illusion of myself emerging from cover to attack to trigger any readied actions.

I would assume blindsight/echolocation would easily see through the illusion because it was stated it is not a solid object.

I suppose with blindsight in general, but echolocation specifically is perceiving the world through sound. If I make a sound-only illusion, would it be possible to make that sound be "what echolocation would hear from a large rock", just like a visual illusion of that rock is "what the eyes would see looking at a large rock"?

It might be a matter of finesse that's difficult or impossible to achieve without having echolocation myself, so I can practice my sonic illusions, but it seems like something that should be possible.

Unfortunately this would only be possible if you had attuned this specific illusion spell to the creature creating the echolocation. A million bats could be screeching and using echolocation all in one clump and they would all still see the image of a rock clear even with all of the sounds bouncing off everywhere because every bats echolocation is different from another. So unless you could find a way to make an illusion that created the sounds of THAT SPECIFIC bats echolocation than its impossible. It could potentially work on some monster with echolocation if we were to assume it used it in a different way than a bats, whales, dolphins, oilbirds, ect.

But what would be the point in creating a monster that is given the random description as having echolocation that is different than any of its own kind.

Why would it need to be attuned to each specific bat? Sure, the bat is making the initial noise, but hear me out:

With a visual illusion, (assuming real-world physics regarding sight) we're making an illusion that light bounces off of, so that the right colors are visible to any observers. I am assuming a visual illusion still casts a shadow, because it seems like something that would be mentioned if not. We don't need to attune the visual illusion to each light source, we just make an effect that interacts with light to give the appearance we want.

I would argue that with a sonic illusion, we would be creating an effect that interacts with sound (vibrations) such that it gives the (sonic) appearance of a solid object, but which solid objects would still pass through as usual.

I dont think you understood what I said, you can definitely make sonic sounds that can be perceived as real. But it will not confuse a bat because a bats echolocation will not be disrupted regardless of however many other sounds are being made around it, because its echolocation is not the same as the bats echolocation next to it. So if you made a sonic illusion without extensive study on how to make a sound that effects multiple bats in the region, you will be extremely lucky to make a "sonic" rock look real to a single bats echolocation. 

So unless you find a way to make the rock actually physical, then the echolocation will just not see it. 

This has been one of the best cantrips to explore creative options. 

Take a look at our creative uses for minor illusion: https://mymswell.com/2019/09/19/clever-cantrip-uses-minor-illusion-dnd-5e/

Quote from GiantReaver >>

... a bats echolocation will not be disrupted...because its echolocation is not the same as the bats echolocation next to it.

 Both bats have the same organs for sensing sound & interpreting that data as a physical shape.

If the sonic illusion can't fool 2 different bats' echolocation, why does a visual illusion fool 2 different humans' eyesight?

Quote from Khayyin >>

Quote from GiantReaver >>

... a bats echolocation will not be disrupted...because its echolocation is not the same as the bats echolocation next to it.

 Both bats have the same organs for sensing sound & interpreting that data as a physical shape.

If the sonic illusion can't fool 2 different bats' echolocation, why does a visual illusion fool 2 different humans' eyesight?

 Ooh, this one I know :)

Echolocation in bats is not just 'seeing with sound' as we might say as a shorthand explanation. Bats can't build a mental picture of all sounds they hear because otherwise something silent would be invisible to them. Instead, the sounds they use to build a mental picture are clicks that they make themselves. The time lag between the production of the click and hearing it's echo tells the bat how far away an object is. 

Compare that to how we see - light from a source reflects off an object and enters our eye. We don't produce the light ourselves.

Applying that to minor illusion, the idea of the spell is that it produces the appearance of an object or a sound. It is reasonable to imagine that this light would be interpreted by any eyeball as light reflecting off an object, however it is unreasonable to imagine that the spell fools an animal into believing it made a sound that it didn't.

Expanding this more widely to blindsight, in general creatures with blindsight could be thought to be immune to a visual minor illusion, but would still be aware of a sound that the spell made (assuming they had some sensitivity to sound, snakes for example have no ears but are aware of vibrations in the ground such as heavy footsteps).

Tldr: bats make the sounds they 'see' so echolocation and sight are fundamentally different.

Ok but what if I create a dangerous sound? For example, if I create the moan of a cloaker, or a sound that would normally do thunder damage, would it hurt the target?

but it's not a verbal spell.. Why should I use its Spellword?If you don't want to reveal your magic it seems to me like a drawback.

could you cast this as an image, then on round two cast a sound to accompany it? say a sleeping guard that snores?