Yes.

And neither | nor # nor < nor > nor & or any other stupid idea.

What will say: Within the ASCII range only [_0-9A-Za-z] shall be permitted, and since case-insensitive [_0-9A-Z].

Well, with exception of existing one-character-functions {{!}} and {{=}}.

Pondering a while I would request for any syntactic identifier core:

• If length 1 then either ! or =.
• Otherwise at least two characters.
• First one is to be a letter, generic ASCII A-Z, or Unicode letter for L10N, or ideographic.
• More letters etc. permitted.
• Case insensitive by legacy.
• May contain a digit, like REVISIONDAY2, or Unicode digit for L10N.
• A _ is permitted inside but neither first nor last, and not subsequent.
• Function names may be prefixed by # which are not part of the core identifier. Those avoid mixing with any template since a page name cannot start with #.