For one, make it qualify for Share Soulmeld automatically.
For two, have it share 3/4 of the meldshaper's HP, and have HD equal to the meldshaper's level (for BAB, feats, skills, etc).
For three, it can move anywhere within 1 mile of you, and can communicate telepathically when within (100 x meldshaper level) feet.
For four, its ranged attack is (10 x meldshaper level) feet, damage is small, but based on the soulspark's level, and is multiplied by the essentia you invest in it (such as, 3 points of damage per Hit Die times the number of essentia you pour into it; thus, at first level, 0 points of essentia = 0 damage (0x1x3), 2 points of essentia = 6 damage (2x1x3), etc).
For five, it should grant bonuses to the meldshaper depending on what soulmeld is bound to its singular chakra. (Any soulmeld can be bound to this unique 'universal' chakra, and the bonus is based on what chakra the soulmeld in question can be bound to, including totem.)
For six, you should be able to share/channel spells and powers through the soulspark familiar at later levels, just like you would with a psicrystal (familiars can't generally channel spells).
For seven, it should either have your skill ranks, or have a decent skill list of its very own (that you can borrow, maybe?).
For eight, the ability to go incorporeal for a limited time per day, dependent on level.
And maybe a few other things.