My desktop MW 1.23.5 shows a particular home page (specified with MediaWiki:Mainpage. I couldn't see the option, but it would be useful to be able to specify:
(A) A different home page for mobiles/tablets. eg. $wgMFMainpage="My Mobile Home Page"
(B) A different home page for different mobiles and tablets (ie. based on screen size)
While I was considering this in relation to Extension:MobileFrontend, I can see no reason why MW itself could not offer this, as people can use in on smartphones and tablets with MobileFrontend.