What do you mean when you say "grouping"? How would this look like in the different tools that consume TemplateData? Does this include some functional changes, or is it really only visually?

Instead of just having a long list of parameters, in the template creator dialog, they could be grouped and those group names would act as separators/headers for their children parameters:

Likewise on the template documentation page, these could be separated by H3 headings or something:

It's just visual

Small note that the sets key exists and it's similar, but not identical, as grouped parameters are grouped "thematically" and not by function: they don't have to be used together to function.

Vorlagenmeister has this feature and it behaves differently depending on whether the group was given a name or not. I've brought some examples from huwiki.

Unnamed groups

<Group>
  <Parameter name="háttérszín">
    <Help></Help>
  </Parameter>
  <Parameter name="betűszín">
    <Help></Help>
  </Parameter>
</Group>
<Group>
  <Parameter name="név">
    <Help></Help>
  </Parameter>
</Group>
[...]

This XML pattern creates this layout:

Named groups

<Group name="Általános">
  <Parameter name="név" label="név" null="false">
    <Help></Help>
  </Parameter>
  <Parameter name="helyi név" label="Helyi név">
    <Help></Help>
  </Parameter>
  <Parameter name="mottó" label="Mottó">
    <Help></Help>
  </Parameter>
   [...]
</Group>
<Group name="Képek">
  <Parameter name="kép" label="Városkép">
    <Help></Help>
  </Parameter>
  <Parameter name="képméret" label="Városkép mérete">
    <Help></Help>
  </Parameter>
  <Parameter name="képaláírás"  label="Városkép aláírása">
    <Help></Help>
  </Parameter>
   [...]
</Group>
[...]

While this XML pattern creates a layout with tabs that scroll to the relevant section: