As far as open source software goes, maybe flatfile CMSs like Jekyll. Maybe newer blogging software like Ghost. These still require steeper learning curves (and sometimes, hosting requirements) than WordPress though, which is why WordPress will probably have the most market share out of any CMS for the foreseeable future.
I’d say its biggest “threats” are hosted platforms like Shopify (ecommerce specifically), Medium (blogging specifically), and website builders like SquareSpace and Wix.
While managed WordPress hosts have helped somewhat, I believe there’s still room for a more intuitive experience, especially during the on-boarding process.