I think the Challenger looks fantastic, there's no question about it. But... It is basically a mid-cycle refresh, just as the Mustangs got in 2010 and 2011. At some point it will get a complete redesign too and it may not look quite so retro any more either.
I just want to know what "theme" a previous poster was referring to that the other Mustangs built from 1974-2004 followed but the 2015 doesn't. Just because the current Mustang has a retro design theme and bears a strong resemblance to the first generation cars doesn't cut it in my book. For 30 years the Mustang never looked anything close to the original or current generations. Yet Ford sold a lot of them. So... What "theme" would that be?
Well, "mid cycle" is in the eye of the beholder (Dodge labels this as a mid-cycle refresh), but the drive train changes (8 speed fast shifting rev-matching auto) and the interior changes (fantastic) coupled with the front and rear view changes make this a rather new car - similar to Mustang in 2011. The attached "Dodge Product Plan" (presented to shareholders) shows a redesign in 2018 - long time from now. As far as the Mustang designs in the period you mentioned, I think the answer is there really wasn't a "theme(I had a 2000 GT vert)." However in 2005 the designers went back to the "iconic image" that originally made Mustang an incredibly popular car in the 60s and with great drive-train technology (Maybe with the exception of the manual transmission).
2013 Mustang GT Premium Convertible
Candy Apple Red, Auto, FRPP Tune, GT500 Axlebacks, KONI Yellow Sport (adjustable dampers), FRPP Front and Rear swaybars, Custom Removable 4 Point Roll-Bar, BOSS 302 Low Expansion Brake Lines, Roush side splitters, GT500 spoiler, Redline Tuning hood struts,18" Ford Chromed Aluminum Wheels (9R3Z-1K007-A), Michelin Pilot Super Sports - 235X50X18
2011 Mustang V6 Premium Convertible - Gone