It would still be based of V35A, it makes sense since the 3.5 is a pretty long stroke, is used in trucks and the LS. Likely would be de-stroked to smaller a displacement.
The best engine possibility isn't Mazda's I6 or their 3.5TT, it's if they make their own I6 from their G16E. That engine is built for performance first, if they add on another 3 cylinders, a 3.2LT I6 easily seeing 400hp+ in base format. Heck they can get somewhere around 600hp if they wanted...