Extension:PersonalisedHomepage

This extension allows users to personalise the home page by creating a new page in their user space that will be used instead.

How to customise
Create a sub-page of your user page which has the same name as the main page of your wiki. By default this is, but this is configurable on a per-wiki basis so it may differ in your case. You can find the correct name by checking the contents of MediaWiki:Mainpage for the wiki in question. " " should be replaced by the appropriate value for your wiki throughout the remainder of these instructions.

If your username was " " you would therefore create your personalised home page at " ". You can edit the page to contain whatever you want. If you want to include the contents of the non-customised main page then add  at the point you want it to appear.

Source
Available on my test wiki.