The following mage paths:
Continuing magic specialization--you may join the Psijic Order
Going to a "sneak" specialization--you may join the Dark Brotherhood
Going to a warrior specialization--you may join the Vigilants of Stendarr
The following thief paths:
Continuing thief specialization--you may become a Nighting Gale
Going to a mage specialization--you may join the Dark Brotherhood
Going to a warrior specialization--you may join the Penitus Oculotus
The following warrior paths:
Continuing the warrior specialization--you may join the Blades
Going to a mage specialization--you may join the Dark Brotherhood
Going to a thief specialization--you may join the Alik'r
Also each specialization would open up unique items only that specialization can receive. You would also have the choice to remain independent. I believe this to be a great way to increase RP prospects. The specializations also do not limit you to learning magic if you war a double specialized warrior, for example, you just have more prowess with the archetype.
