Maybe he should focus on the basic animals first. no bats, birds(well theres chickens, but like eagles and stuff like that)and-as benefiits stated- visible fish. notch...really?
Hypothetical: You read a thread in the Suggestions forum. You suddenly realize you've seen the idea before. Maybe it's a duplicate? No. Maybe it's just you? No. Maybe it's been done already? Very likely: Click here if there's a mod for that.
What if it was attracted to light, like torches.
And it would go and eat your torches, then it would glow.
Maybe slims would go around the caves and eats the torches, making it dark again, and all you can see is the glowing slime. After you kill the slime it drops the torches, or glow stone powder.
CLICK HIM
A specific idea, details, explanation, purpose...You know, the BASIC COMPONENTS of a suggestion.
A mob that glows wouldn't really serve a purpose. If you come up with a good use or purpose for one, then maybe.
And it would go and eat your torches, then it would glow.
Maybe slims would go around the caves and eats the torches, making it dark again, and all you can see is the glowing slime. After you kill the slime it drops the torches, or glow stone powder.
Just an idea.