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