There's absolutely NO possibility for a link to start with a colon, as it is invalid in every page name (on all wikis, not just those from Wikimedia).
And there's absolutely no point at all in changing that to a plain-text with visible brackets surrounding the text with the multiple leading colons, and no point to enforce it (badly) by silently inserting nowiki tags, which also obscures the wikitext and make it even less editable.
Why do you want to see "[[::" and "]]" as plain-text ? If one really wants to see that, the "nowiki" tags can be added manually to escape them **only** where this plain-text is expected (extremely rare case in fact, compared to the very common cases where extra leading colons may be inserted by templates using optional parameters which may be empty for an optional namespace indicator or interwiki prefix).
Leading colons are used explicitly to force the interpretation as a wikilink and not as a rendered image or categorization metadata, or to make distinction between template names in the template namespace (no leading colon) or a transcluded page from another specified namespace or the root namespace.
This new requirement removes that useful distinction and just breaks many pages and complicates a lot the development of templates by forcing them to inspect the values of substituted parameters (we need now to add various tricky "#if" tests, the expansion time or nesting level is increased, the number of expanded nodes increases by forcing parameters tro be expanded multiple times... in summary this adds additional charges on the server or makes some page impossible now to render correctly due to resource exhaustion caused by these extra tests).
So in summary I do not like this new requirement that just breaks things and makes things just more complicated (and does not even help other parsers to disambiguate things. For me any wikitext sequence matched by this regexp (except those found in "nowiki" sections, or in HTML comments normally stripped in an earlier first stage of the parser, before handling "includeonly", "noinclude" and "onlyinclude" sections in the second earlier stage):
\[\[[ \t]*:*[ \t]*([^:\|\[\]][^\|\]]*)[ \t]*(\|[^\]]+)?\]\]
is a wikilink (or interwiki link) whose target is the page indicated in the first regexp-grouping parentheses in blue (to render as an HTML link with inline text content) if there's 1 or more leading colons (after stripping ignorable whitespaces , just indicated as [ \t]*
in this simplified regexp, as there are other ignorable whitespaces), and the content of the second regexp-grouping parentheses in green is the inline content to render in that surrounding HTML link, independantly of the number of colons (indicated in red).
Note that the content matched by the blue group above may include transclusions of templates (or expansion of magic keywords) surrounded by {{...}}
; their expansion could return leading colons to discard silently as well if they are in excess and there's already at least one colon before them...
Only when there's no leading colon at all (in red, or in the expansion of wiki templates in the blue group), the target may be interpreted as inline file rendering (image thumbnails, or audio/video player objects), or as a categorization (when the content of the first regexp-parentheses pair starts by the special namespaces names for files or categories); and otherwise it will also generate an HTML link with the inline content of the second group displayed.