Here's the suggestion:
At every even level (when they gain a spell level), give the Sorc a choice of a new Spell Known from the bloodline list OR a Transformation. That way a Sorc who gained his powers from a pact with a Fey Prince is different from someone whose dad got freaky with a Nymph, and is different from someone who got his power by being born in a Fey Glade.
It can work for all concepts. For example, the Destined Sorcerer might gain luck bonuses to different things or "behind the curtain" powers like Fortification or the ability to ignore miss chances as his Transformational aspect, but gain spells like true strike(1st), misdirection(2nd), heroism(3rd), charm monster(4th), contact other plane(5th), transformation(6th), limited wish(7th), mind blank(8th), foresight(9th).
The level 20 power can still be just a Transformation.
PS. The reason to get a spell at every new spell level is because Sorcs have a huge incentive to PrC into something that gives them a new spell every time they gain a new level so that they have a choice of two spells to cast with their best spell level.