What I think a Retro line should be is a small list of the original cast of the old cartoon the TMNT, Splinter, April, Shredder, Krang, Bebop, Rocksteady, a Foot Soldier, and Perhaps General Traag just to even things out a bit in the exact looks they had in either the very first episodes they had been in or during the later season of the show in the first season 8 episode "Get Shredder", I think the characters look top Notch around this time or a little before in the end of season 7.
I'd say give it a chance if it's successful try with a few more characters such as Leatherhead or Slash as they had been in the old toon style for 2 more series.
The title Retro is given because it is marketed towards older fans who grew up with the series and the casual viewer who goes wow krang or bebop I remember them from when I was a kid I want that it looks just like the show and is better than the one I had as a kid, which will happen as let's face it adults want quality items.We're talking retro for everyone but when it comes down to it, it's the adults who would buy these even on a whim.
But you really never know who's going to buy things and I mean no one does not even people who think they know, They want to make TMNT variants make ones like these ideas and not only will you have your variants but you'll be making allot of older fans happy as well and not only that but they will be much better updated versions hopefully done by Neca or by Playmates if they realize that today's action figure market is allot different than it used to be.
By that I mean that figures are very articulated for the most part or have very detailed sculpts and 9 times out of 10 look exactly like the characters they are based on and are in the correct height/ Scale with every other character in the series they come from.
I really can't stress that there is no better time than now to do this with the turtles anniversary around the corner, I feel if this is passed up now it may never happen.
The images are official style guides used for the 80's-90's cartoon series as well as some screen caps as examples of what I'm talking about.