hm... great question! Lets consider templates for a moment, applied to symbionts.
Symbionts don't really have a specific creature [type], other than symbiont (well, if I recall, some might be aberrations, some fiends, but I could be remembering wrong). But we can assume they are atleast living, corporeal creatures. So, which templates apply to living, corporeal creatures? Other requirements will be listed:
-Half Elemental templates from Manual of the Planes, Req. Int of 4+.
-Half Celestial template from Monster Manual, Req. Int 4+, non-Evil alignment.
-Half Fiend template from Monster Manual, Req Int 4+, non-Good alignment.
-Draconic template from Races of the Dragon
-Half-Dragon template from Monster Manual and Races of the Dragon
-Half-Fay template from Fiend Folio
-Shadow Creature template from Manual of the Planes
-Psudonatural Creature template from Manual of the Planes
-Revived Fossil template from Libris Mortis, Req. a skeletal system
-Skeleton template from Monster Manual, Req. a skeletal system
-Zombie template from Monster Manual, Req. a skeletal system
-Dustform template from Sandstorm
-Chameleon template from Underdark
-Faerzress-Infused template from Underdark
-Mineral Warrior template from Underdark
-Phantom templates from Monster Manual V
-Ice Beast template from Frostburn
-Lolth-Touched template from Monster Manual IV, Req. non-Good, non-Lawful alignment
-Saint template from Book of Exalted Deeds, Req. Good alignment
-Sanctified template from Book of Exalted Deeds, Req. Evil alignment
This list doesn't include templates from magazines, nor does it consider templates that can be applied to undead, after a corporeal creature is made undead by one of the abouve listed templates that grants undead-ness.
Now... how to actually apply these templates? Magic? Selective Breeding? A very old Fleshwarper NPC who's devoted his life to tinkering with such things? Dunno, good luck.
Hope this helps!