It would be awesome if the WP search had a way of easily extending the searched fields (like the Search Everything plugin for instance), but also give the blogowner the option to add (multiple) advanced search-forms (fields).
This would give blogowners the option to add searchforms for specific parts of the website, ie the default search form (one input) for a blog, a more advanced form for a portfolio (default input for keywords, plus services selection) etc.
Another example: I am working on a new version of my website wpontwerpers.nl, which displays accounts of WP designers/developers in the Netherlands, Belgium and Luxembourg.
The website has a few sections: a blog, the accounts, normal pages and in the future I will add a few other sections (ie a marketplace).
It would be awesome if I would be able to just add fields to my advanced search form page (for the accounts post_type) with predefined names (ie the slug of the term/taxonomy as the fieldname) that would automatically be applied to the search query (instead of having to add the search query for it manually).