It would be really useful if patterns could have access to template parameters.

The parameter would still have to be declared by the template for it to be visible (as with tunnel parameters). Also, when visible, it would shadow any global variables with the same name (just as with local variables/parameters).

For example:

<xsl:template match="*[. is $node]">
  <xsl:param name="node" tunnel="yes"/>
  <found/>
</xsl:template>

<xsl:template match="*">
  <not-found/>
</xsl:template>

Right now I have to use <xsl:choose> (yuck).

I can't think of any issues with this other than the somewhat unintuitive aspect of the parameter being declared just below where it's being referenced (as opposed to above).