I rather liked the suggestion of having tentacle barbs and hooks and similar on appropriate body parts. You could use the [number:x] tag to quickly and easily define them the same way teeth are. There's actually a bodypart similar to that already that you can copy and rename;
[BODY:TENTACLE_TEETH]
[BP:TOOTH:tooth:teeth][CON_CAT:TENTACLE][CATEGORY:TOOTH][NUMBER:30][SOCKET][SMALL]
[DEFAULT_RELSIZE:1]
this way the teeth will be applied to any other bodypart that has the category TENTACLE. You could change that to any other category in fact, having one for hands, arms, or whatever else you want to put 'teeth' on.
This way you can make things like cephalopods(squid and octopi) or giant insects like mantids that have sharp points on their arms made of a different material and used in an attack. This way you can have some dangerous predators like you said you wanted. Not unnatural, just not a normal mammal/reptile adaptation. I'd go as far as allowing thing's like Half Life's Bullsquid be generated. Actually I may mod them in at some point, just because they're awesome.
And I think you really should define different types of tails and have them selected separately from limbs; you could then give large prey animals larger tails with an attack based off them. This was common among dinosaurs, so it is theorized. Prehensile tails are of course natural, and you could define a very small tail for mammals like bovids and equine. Stingers and tail ornaments could be attached only to the normal or large variations. The [DEFAULT_RELSIZE:1] affects how difficult it is to hit that part when attacked, and that has a profound effect on combat. I think it also affects how often it is chosen as the target of an attack, versus other parts. Scorpions and lizards should have larger tails.
Are you using the [BODY_DETAIL_PLAN:x] token and defining the plans in a file in your program folder? (b_detail_plan_default.txt for unmodded DF) it's used to modify tissue layers and body sizes within a specific creature. That way you can change the default_relsize of a spider's limbs to make extremely heavy spiders like tarantulas with large abdomens or extremely thin and gaunt ones with long legs and tiny bodies like orb weavers, or other combinations etc... just by attaching it to a new detail plan, instead of making new body parts. You can also change tissue thicknesses that way.
Also, be sure that insects and arachnids are anatomically correct; the legs should be set to attach to the upper body(thorax, cephalothorax) not lowerbody (abdomen). You could combine them together and have an inert abdomen part hanging off it, but losing the abdomen is essentially every bit as fatal as chopping a human in half below the ribs, so I'd leave them separate and just attach limbs to the upper body.
Fluff like the frills/shields and things should be encouraged and named differently from normal fins; it's not a large amount of information for the game to track, and it makes the creature far more interesting to players. It would still be easy to base them entirely off the fin entries in the raws. Again, use the [NUMBER] tag where there are many of them attached to the same part.
I like the symbol idea for names. I actually intended to suggest that before I read it.
You could combine it with the language files to produce names in the language of any of the races. I noticed while typing my suggestion that I couldn't find the symbols list on the wiki anymore. I intend to change that if I can.