A what-if certainly benefits from a good back story. Make it sound credible, even if those who know anything will know that it is not "real".
Here's an example of a plastic modeler's what-if that I saw recently:
[link=http://www.flickr.com/photos/dizzyfugu/8153758178/]
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[link=http://www.flickr.com/photos/dizzyfugu/8153758178/]1:72 Vought-CAC Corsair S(AW).1, '236'/WZ882, Royal Australian Navy, 806 Squadron/HMAS Melbourne, 1957 (Whif/Kit conversion)[/link] by [link=http://www.flickr.com/people/dizzyfugu/]dizzyfugu[/link], on Flickr