Apple – they already have a crawler out indexing.
Yahoo/Verizon – Verizon is the parent company of the once mighty Yahoo. A lot depends on Google losing exclusive default search engine on Android verizon could step in for phones sold by Verizion. Yahoo controls a lot of old search engine brand names.
Samsung – largest maker of Android phones. Again, if Google could no longer dictate default search on Android Samsung might try it.
Amazon – they know how to crawl the web. Long shot.
DuckDuckGo – right now they rely on Bing for most of their web results. It’s a longshot but they might want to build their own index.
Meta – parent co. of Facebook.