See
User:Jubileeclipman/List of problematic 21st-century composer articles for a full listing of 21st-century composer articles (work in progress...). Other listings will be available when the other major list-articles are attended to. See the WolterBot cleanup listing below, too, for other critical issues.
Quartal and quintal harmony: Confusion of definition and (apparently) irrelevent material; also, musical examples are often off-topic or otherwise unhelpful.
Microtonal music: tagged with cleanup and personal reflection. Also contains a certain amount of apparently irrelevent material which needs to be better integrated (eg Enharmonicity)
Hans-Christoph Steiner, composer of
computer music who created the score that our picture is an extract from (though mainly a software developer specialising in sound production
[1])
Jacob Ter Veldhuis a very important Dutch composer who, surprisingly, doesn't yet have an article.
Individual composer articles - add list of works for each (organized chronologically or by instrumentation); or, if the list of works is very large, create separate articles such as
List of compositions by Thomas Adès.
Individual composer articles - find and add freely licensed photos for all composers; this may involve sending emails to the composers or their estates.
Individual composer articles and/or articles about individual compositions - find and add freely licensed or fair use images of particularly notable or interesting score pages, such as
Image:TidesOfManaunaunOpening.jpg, which is in the public domain.