Default templates should use user's indentation settings.
For now, only tabs are used, and seems there's no way to set default indentation for templates.
(For now the user can edit the files by hand.)
I'm not sure whether to implement this because it could cause problems. The default templates are created on first run of Geany (or in future, installed with Geany). So they have the default indent pref.
When using New with Template, Geany could convert all tab characters into the correct indentation - but what if the user had created a file template with Tabs & Spaces indentation? Geany's attempt to fixup indentation would then break the Tabs & Spaces template contents.
We could insist that template files use tabs-only (or spaces-only) indentation, but maybe people already have templates with Tabs & Spaces indentation. Although it might be worth breaking this support, we would have to be careful - maybe be we could only fixup indentation if there is no mixture of tabs and spaces.
Another option would be to have a pref, off by default, to fixup template file indentation.
Hum, well, I haven't seen it this way…
I think the better proposal is your last one, an automatic conversion following a pref.
Sounds good for me, this provide what I would have and let currently happy peoples away from this change.