Here are some interesting things to mull over if we want to make The Hobbit as Prequel.
(The following list is by no means exhaustive)
(1) Motive. Why does Gandalf send Bilbo off with Thirteen Dwarves? To kill a dragon who might be enlisted by Sauron in his War? Frodo is sent off to sneak into Mordor and secretly destroy the Ring in the Mountain of Fire. Bilbo is sent - to do what exactly? To steal something, make Smaug mad enough to fly to Laketown and get killed. Such foresight surely must fail Gandalf! If Gandalf has this foresight - well, then the idea is ridiculous! The plan would need Bilbo sneak in and kill the dragon, or sneak in and discover a means of killing Smaug? Why pick Bilbo? What has he shown Gandalf which makes Gandalf think him capable of such brilliance? Also, the viewer must know early why Bilbo goes. Need a pretty good reason for a LotRized tale.(Yes, perhaps it is far fetched he go as a Burglar, as in the book, but not in a childrens tale as opposed to a grown-up tale, especially if burglary is the only reason he goes).
(2) The dwarves turn up AS EXPECTED. Bilbo surely knows the Plan!
(3) Trolls. Total rewrite required. The dwarves get captured by three trolls so readily. Big battle? Does any dwarf get killed? Does Gandalf trick them to forgetting daylight, which knowledge their existence depends on, and has always depended on! A big battle will do it, I guess.
(4) "Tra la ally." A serious bunch. Etheral singing allowed - but with new (Sindaran) words? Serious business (but loss of color). Bilbo can't be awestruck by the Elves and Elrond's Lore. Surely, Gandalf would have filled him in already about the world and dangers the little fellow faced on the quest?
(4) Stone Trolls. Nah! Just a storm with boulders tumbling. (Loss of color?)
(5) Goblins. No song. Dark, dark, evil goblins. Ambush? No cracks snapping shut? Too magical! (Lack of color).
(6) Gollum. No riddles. Ridiculous in an adult movie! You can't possibly have a serious riddle game, can you? (Serious enough for adult tastes, I mean).
(7) Goblins and wargs. No songs. Jeering as they light the fires beneath the trees, but of course! Very LotRingish. (What about the loss of color?)
I'll stop here (at least for the moment). The Hobbit takes you on a humorous but nonetheless thrilling and magical ride ("Prepare to suspend your disbelief!") Beware The Hobbit Prequel! Beware the attempt to make it seem realistic! Won't work!