MkT wheel base, but in sedan form...that would be one interesting car for the "near luxury" set...I honestly think there would be a market for that in the livery/taxi fleets as well as selling 2-3K a month retail....especially in South Florida and like regions. Lincoln is headed to the stratosphere in market terms, but Mercury can attract the livery/taxi market with Marquis and sell it well without hobbling Taurus or MKS sales.
Just for numbers sake, I looked up the specs as follows:
Taurus (2010) : overall length - 202.9 wheelbase - 112.9
Grand Marquis: overall length - 212 wheelbase - 114.6
Town Car: overall length - 215 wheelbase - 117.7
Town Car L: overall length - 221.4 wheelbase - 123.7
MKT: overall length - 207.6 wheelbase - 117.9
A D3 based Marquis can consolidate current Grand Marquis, Town Car, and Town Car L sales once Panther is put out to pasture. D3 has shown the (pardon the pun) flexibility and can be summoned up for production in St. Thomas Assembly as Panther winds down.
Having Mercury in the fleet market would be a boon to Ford as a way to differentiate both marques (Ford and Lincoln) from a fleet stigma.