The latest word is that the next specialty mustang will be a new Bullitt edition. And the word on that is that it will likely pack a 4V variant of the current 3V engine. I've heard no concrete plans to put an NA 5.4 under the hood of the mustang.
That would be nice if they mass produced a new BOSS.
There are tons of rumors about the new BOSS Mustang.
Sooner or later Ford will slap the BOSS name on a Mustang
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In my stable:
1972 Grande 302
1972 Convertible 351C
1992 GT Convertible
2001 GT Convertible