That's exactly what they did.
Etna hardware - LTE + Droid X shell = Droid X2
They couldn't use the exact Etna form factor for a 3g phone or something for another carrier, since they had already designed and marketed it for Verizon's Bionic. Its too bad they didn't repackage the Targa into that shell, but it probably has something to do with that shell being advertised as having an NVidia processor, while the Targa has a TI.
Not to mention that the Targa was already a separate project with its current form-factor already in place. They already admitted that all they did was fast-track the Targa's development, rather than actually redesign the Etna, so a major change like that would likely not be possible with a shortened development schedule.