How to tag a bollard with a certain shape ?

The tag "barrier=bollard" has already a sub-key called "bollard=*" documented on the wiki.
At the moment, only two values have been specified "rising" and "removable". You can also use the tool "taginfo" to check the current list of values set by the contributors. As you can see, some values are inconsistent with the current documentation like "wood" which does not say if it is "removable" or "rising" or not.

I guess the best would be to split the current sub-tag in 3 sub-tags : one saying if the bollard is removable or not, one describing the shape (pole, pyramid, etc) and one describing the material (wood, stone, etc).

I would suggest to keep the current "bollard=" for "removable" (for the ~3000 tags already in use) and reuse the existing "material=" tag for wood, stone, etc.

Then propose something new for the shape e.g. "bollard:shape=pole | ball | pyramid | etc". But you should submit a formal proposal on the wiki first then call for comments on the tagging mailing list (subscription required) (archives). The full process of approving a new tag is not really required here since you just go at a very low level of details.

You could also use the new tag directly with your favorite editor. But formalising your tag proposal on the wiki increases its chance to be reused by others.