I have already discussed a more flexible avatar for humanoid/quadraped forms with somewhat more flexibility than present here. Below are some ideas for some other generic avatar shapes. Obviously one could never cover all possibilities, but the below -- hopefully -- cover very many. The advantage of having a small number of very-flexible built-in avatar forms is that you don't run into the "you need a plug-in to see my avatar" issue as well as meaning animations can be much more reusable (while not all radoid animations would look good on all radoid forms, they would at least all work to some degree). I recognise this doesn't eliminate the need for purely freeform avatars as well, but covering the most common cases with highly mutable standard forms is going to be more efficient from a potential-reuse perspective. Fully custom avartar forms can still be implemented on top of this, of course.
This is the LindenLabs avatar, available for backwards/cross compatibility.
Simplest of all possible avatars. This is a cylinder with a sphere on top, representing a body and a head. No articulated parts. Useful for where speed is more important than realism. Only two dimensional attributes: diameter (same for head and body) and body height (which if zero, results in the avatar being only a sphere).
Bonus, compatible with any animation as it just uses the position and angle of the base bone and ignores the rest.
You could probably do some really cute stuff with these. Also useful with very small children where the uncanny valley tends to be more pronounced.
Also, a good simple base avatar if you want to do extensive prim-attachment based avatar building.
An avatar shape based on the MakeHuman project's work. This is a much better-thought-out mesh with more detail and realism for not much more mesh-complexity. It is a single generic mesh which then has modifiers applied to it, so the same method as the SL avatar system but better thought out. I would prefer to stick with a suitably adapted SL-like texture map as the MakeHuman texture map is a bit scary-weird for people who can't think up-side-down and also doesn't look so good for using generic flat textures as clothes.
The MakeHuman project is GPL v3, so I am not sure how that would work with a SL-based third-party viewer - definitely couldn't be upstreamed back to SL, but does that really matter? The SL mesh is still supported as mentioned above.
To create a range of squid, octipi, starfish, jellyfish. Can have from 2 to (say) 12 legs, which are arranged symetrically on either or both upper and lower body. Special legs (eg squid) might be achieved with attachments. Mouth is assumed to be in the center of the base or the centre of the top of the head, with one jaw segment per body segment. Number of eyes defined separately for upper and lower body (0-12, must be a factor or multiple of number of tentacles) and arranged symetrically around the body (position definable from tentacle tips to near the top of the body and from centre of segment to edge between segments). By reducing tentacle length to zero and having a long body, a snake-like forms can be created. Possibly some extra detail possibility around the top of the body to allow it to become the head of a wyrm, snake, eel, slug/snail, sandworm, Goa'uld. ;-)
Using the above body-texture template, for an Octipus, the texture would be repeated 8 times, with the upper legs retracted to Zero. For a snake, the lower legs would be retracted to 0, the lower body tapered down to a sharp point, and the upper body repeated two times with the upper legs retracted enough to become the jaws. A sandworm would be a snake with three jaws, a Goa'uld 4 repeats. A slug would be like a snake but with two eye-ended tentacles instead of jaws made from the top tentacles.