Familypedia.wikia.org was founded in 2004, using software almost identical to Wikipedia's, and started using three standard semantic extensions in 2009. By the end of 2020 it had about 270,000 articles and 390,000 other pages.
Familypedia has a well-developed system of templates, which can display on an individual's article numerous data items and tables of spouses, siblings (differentiating half-siblings) and children, with subpages for automatically-generated ancestry trees and descendant lists. Example at http://familypedia.wikia.org/wiki/Heather_Campbell_(1911-2004)
Numerous other tables are available, notably tables of people who had events such as birth and marriage in a specific locality or county or nation. Example at http://familypedia.wikia.org/wiki/Plymouth/bdm
Searching can be on any combination of semantic facts ("properties"). A complex example is at https://familypedia.wikia.org/wiki/Semantic_MediaWiki/demo_query-subquery
Intending developers of a new semantic extension for genealogy can probably learn a lot from Familypedia. A new semantic extension may not be needed but could be worth pursuing. This one seems to have been going very slowly, which is a pity. [[Extension:Genealogy]] is newer and may be more likely to succeed.