I really love this thread! Great ideas in here... just a few things that jumped into my mind:
The Jedi shouldnt be portrayed as some new-age hippies! Yes, of course, they are spiritual and all of that, but they do kill, if its necessary, etc... They are "Jedi Knights", we should remember this and maybe even find some more analogies to medieval knights. That way Obi-Wan could easily become the Knight of the Organas and thus he becomes a General.
I dont think Obi-Wan should ever leave the Jedi Knight. I don't even think we should approach the Jedi Order as some sort of organization that you can leave or get into. Its more about the formal training and calling of the force.
I really love most of the ideas posted by Blackhawk and Chainsaw, so many of my thoughts build up upon them:
Blackhawk was talking about an invasion of Alderaan. Now, what do we do with that? I think the planet could be taken "hostage". The republic should be built upon trade, so the invaders would blockade the planet, threaten to destroy industry or as a last measure kill off population or the royal family.
The republic senate could deicde to give military aide to Alderaan, because they won't let themselves be blackmailed. Since this could lead to the death of millions Obi-Wan tries to prevent a massacre through diplomacy and travels to Alderaan. Thats when the prince of Alderaan, Bail Organa, flees from his responsibilities and Obi Wan now has to track him down, because the royal house has to stand together as one to make a treaty.
My biggest problem with this approach is that it sounds a bit too similar to the beginning of TPM. Especially the first part with the occupation. Someone made an important observation in this thread, that we should try to do things as different as possible, in comparison to how GL did them in the PT. So the conflict around Alderaan should be very different. No Droids. No big invasion. Maybe go more with the clone concept and have infiltration? Replacing people in key positions, slowly hiding a military force on the planet, etc. But: We dont need to see this, this sould happen before NPT, and could be told to the viewer through dialogue (for example Obi-Wan explaining the situation to Anakin, who has no idea what he has gotten himself into...). Important for us is not the conflict itself, but the characters, obi-wan, bail, anakin and how the eventual resolution of this first Clone War begins to corrupt the Republic and shows signs of its possible demise.
We all agree that Episode I should stand on its own, right? So what we really need is an adventure that MUST be fleshed out. Whats the story we want to tell? In ANH its "Rise of a hero and victory in an important battle against the Evil Empire". We already have the "Rise of a hero" part and i can imagine most of it just fine, but we need real conflict, loss, drama, not just some obscure or far away Clone War (in ANH lukes next of kin are killed, so the empire is a real threat). ANH has all those little scenes that establish scenario and characters (like the cantina, garbage smasher, etc)... there's so much happening, its exciting to watch.
Oh and by the way: We dont need to explain everything right away. Leave some mystery to it. We dont have to explain exactly how the Jedi are organized, how many there are... Of course we can already mention Yoda, but we dont have to explain why he's on Dagobah or what he's doing there. These are all things that can be done in Ep2 and Ep3. This will keep people hooked and make it more interesting to watch.