I agree. Mheetu would have been a fun character, and I personally believe it's a shame they excluded him.
I know he was never supposed to play a big role, but if nothing, he could be useful for developing Nala's character. In early scripts it's said she protected Mheetu from the hyenas during Scar's reign, and that sounds just like Nala to me: brave, clever and responsible.
I believe he might have been something akin to Michael or Danny from Peter Pan... just slightly younger then Nala and Simba.
Speaking of which, I wonder if this little one could be Mheetu by some odd chances. It does seem a bit peculiar that whoever was illustrating the comic just decided to throw in a random baby lion. Then again, we don't even know it's Sarafina for certain.