Welcome to the site sambo77! I'm going to inform you on a few different things.

1. the 5.7L HEMI costs less to build than the 4.7L engine currently does.
2. the price of a 5.7L powered Challenger could be significantly less if they get rid of a few standard luxury options that they offer with the Charger R/T standard such as leather interior, power seats, premium radio.
3. If they offer a V6 they will probably offer the 3.5L or the 4.0L. The 2.7L is underpowered in the LX cars due to having all its power at the high end, and even if they added technology such as Dual-VVT, it would still not be enough. The price for 2.7L engines is high due to the high demand for that engine for replacements in LH and Cloud cars (Stratus & Sebring sedans) due to the oil sludge problem. I would tend to doubt they would use the 3.7L. They might not have the capacity for another vehicle because they just used the 3.8L V6 in the new Wrangler, instead of the 3.7L.
4. An autostick is no substitute for a manual option. If people want a manual, they want a full fledged manual, now a autostick is a good idea to have as an option instead of a manual.
5. A modified version of a 4.0L V6 could produce 300hp. Some Japanese's manufacturers produce smaller NA engines that produce around 300 hp out of a smaller 3.5L V6. This might be less expensive.

Don't take those things listed as I'm putting your idea down. I just wanted to inform you on a few different things.