The universe needs more space cowboys and samurai. With Disney planning multiple Star Wars movies every year, spin off is inevitable.
Why: The easy answer is "because it's already happening." But the real answer is because Yoda is an enigma. He is the last of his (never named) species, and a movie about the Jedi Master's younger years would shed light on a dark corner of Star Wars lore. Did his people slowly die out? Or did Yoda witness a genocide 800 years ago in which he lost everyone he's ever loved?
Why: Because his story is just as interesting as Luke Skywalker's. He joined the Rebels when his girlfriend was murdered by Imperial forces and became such an accomplished pilot, he is the only pilot (save our heroes) to survive the destruction of both Death Stars. After the original trilogy, he was responsible for bringing down the shields on Coruscant, allowing the Rebels to retake the planet city. He went on to become the greatest pilot in the fleet, getting into all sorts of adventures ripe for movie magic.
Why: Because it's Samuel L. Jackson with a LIGHTSABER, jeez! Need more of a reason? OK, how about Mace Windu was an orphan who was given to the Jedi Academy at the age of six months and was basically raised by Yoda. He created the only purple lightsaber in existence based on a vision and willpower. He created the last form of lightsaber combat, designed specifically to channel his dark tendencies into light. Of all the Jedi who used his stance, he was the only one not to fall to the Dark Side. He is also the only Jedi to beat Palpatine in a duel. Ever.
Jabba the Hutt
Why: Because space gangsters! Despite being a hedonist slob, prior to his death Jabba Desilijic Tiure was the most powerful crime lord in the known galaxy. His underworld empire was so vast, even the Galactic Republic went out of their way not to displease him. The Hutts are basically a Mafia culture, with double-crossing and murder commonplace. But Jabba began his life as a small-time crime boss, so what is the story behind amassing such power unfettered?