Currently there's no way to use [list] without any symbol (which is useful for indents). You're teased to do something like this...

[list]This is my indented text[/list]

...but that will render invalid HTML (as <ul> doesn't allow plain character data and expects a surrounding <li> at least).

So I suggest to also use the type none for lists. To do so, only 2 files need to be modified:
Open /includes/bbcode.php and find

		else if (preg_match('#^(disc|circle|square)$#i', $type))

replace with

		else if (preg_match('#^(disc|circle|square|none)$#i', $type))

Open /includes/message_parser.php and find

			'list'			=> array('bbcode_id' => 9,	'regexp' => array('#\[list(?:=(?:[a-z0-9]|disc|circle|square))?].*\[/list]#ise' => "\$this->bbcode_parse_list('\$0')")),

replace with

			'list'			=> array('bbcode_id' => 9,	'regexp' => array('#\[list(?:=(?:[a-z0-9]|disc|circle|square|none))?].*\[/list]#ise' => "\$this->bbcode_parse_list('\$0')")),

Furthermore CSS2 types should be allowed. For these the function bbcode_list in /includes/bbcode.php needs to be augmented with more ELSE IFs for

• lower-greek
• hebrew
• decimal-leading-zero
• cjk-ideographic
• hiragana
• katakana
• hiragana-iroha
• katakana-iroha