Monk
Psionic Striker
Monks tap into the psionic potential that rests within their bodies and minds, turning that energy further inward to fortify their bodies and sharpen their minds.(italicized because I copied form the link above, I think this best briefly summarizes the Monk)
Psion
Psionic Controller/Striker
4e incarnation of the 3.5 Psion, overly summarized by myself as "the Psionic Wizard".
Empath/Telepath (Telempath?)
Psionic
Leader/Controller
Focuses on mental/psychic abilities that harness the presence of others (allies and enemies alike) in the room. It could be simple enough to just to make it a Leader/Controller with buffs/debuffs that key off allies/opponents as the source. (sharing pain, use psychic abilities to detonate stunning "mind bombs" in the enemy, transferring debuffs from an ally to his opponent, so on and so forth) You could make it complicated, and irritate everyone by giving power sources a game effect. Like, as an Empath, you could harness the natural Arcane energy around those of the Arcane power source, and fire off "magic missile" like abilities. But if the same power/ability is used on a Divine character, it's a radiant bolt instead. I can't think of an easy way to do it, but you'd essentially create a "jack of all trades" that is only limited by the abilities of his party. (coupled with a balanced 4 man party, one of each role, from each of the first 4 power sources, this could be a wickedly sick "perfect 5th member)
Eh, kinda went overboard there. I can kinda see "Empath/Telepath" both being branches similar to those the Sorcerer has access to, but have no idea what you'd call the class itself.
Ninja
Shadow Striker
Fill the class with traits of the "traditional fantasy ninja" (everything from rogue-like stealth strikes to shuriken throwing to disappearing in the shadows) Add in "shadow dancer" abilities, and you could develop a class that is adept at ricocheting around the battlefield dicing up opponents while never giving them a clear target to hit. (like, this is the class you'd choose if you wanted to emulate a 4e version of X-Men's Nightcrawler, but could also be built to be an assassin who is simply "more stealthy" than a Rogue) Poison abilities, martial abilities both armed and unarmed, "the works". A class that allows most any "ninja" archetype. (within reason)
Elementalist
Elemental Striker/Controller
Draws on the powers of Elementals and the Elemental Chaos they call home. This class is like a walking "wild card" of Elementals. Using the first Monster Manual Elemental entry (begins pg. 104) as an idea. First one is a Firelasher. So the Elementalist as a "Flaming Whip" power (Fire Lash), and various Fire/Cyclone powers (stable storm burst centered on the PC, as well as a smaller "stampede" attack that functions by barreling through the opposition) Going on to use the Earthwind Ravager as another example, the Elementalist might have the ability to create a dust storm, or a real "buffeting blast" throwing large stones at the opponent. Instead of making something that people will claim ought to be Arcane, we make something that literally harnesses the elements of the Elemental Chaos the same way Elementals themselves do. Mix and match elements in a way that exemplifies the "elemental chaos" the PC draws its power from.
Okay, it's getting late, so I'll drop some shorter "entries" to show some of my thoughts, hope to get input from everyone else, and build this into a "list of strong potential homebrew classes" at the very least.
Fatespinner
Divine Striker/Controller
Follow the will of Fate or bend Fate to your will. Possibly a Divine class that does NOT need a patron deity, as in, someone who might draw power from the Astral Sea instead of a 4e God.
Archer
Martial Striker
Think Ranger, and then take out everything melee, and take out or reflavor or tweak everything that screams "Ranger", and add in everything that says "Marksmen". More or less, a Fighter with a bow, but not a Ranger. (I've heard others complain about this, so we give them something that is not a Ranger, but doesn't stick them with Fighter powers) Side thought, I wonder if it would be possible to make a Ranged Defender? Maybe an ability that hits, and as long as the opponent is farther than "x" squares away, they grant you a bonus to attack, forcing them to wade through YOUR team's strikers to get to you, or be decimated from a distance. Take the idea of "the Defender", protecting your teammates by, well, "being in the way", and completely rework it.
(sigh, those weren't near as "short" as I'd hoped for the last two, oh well)