1) First, you probably want to split the contents of the main page into a set of templates, to make it modular. 2) Then, you can simply make multiple subpages of the main page each hosting a different selection and presentation of the same content. 3) Finally, to split users across the different versions, you can: a) use a constant title, say [[Main Page]], in which you {{alternate}} the transclusion of each subpage (version) daily or weekly (whatever you can easily tell apart in your stats tool); b) actually send users to the different titles with a random redirect e.g. via JavaScript or ("traditional" approach) some parser function magic either in [[MediaWiki:Mainpage]] or in a #REDIRECT on the [[Main Page]] (results rather unpredictable, depends on cache).
Nemo