I seriously doubt they are going to bring Rain back. Hey, I liked the guy in MK3, but he was only created to sorta replace Raiden. Not story wise, just special effects. The god of thunder is not able to help, wait what this a ninja that can summon a lighting bolt suddenly appears.

The argument of two many ninjas i think is no longer a true argument. Now ALL characters have their own look so actually we don't have too many ninjas as we had in previous Mortal Kombat.

In the time when earlier mortal kombats were made they were of a low budget so palette swapping was a cheap way of introducing many characters. Remember that MK1 was a side project that give the great game it is today.
The success of MK1 was the digitalized characters that gave MK wings to fly until today.

Now today things are different and i don't mind if Rain would be back in a unique way. Why not? And with a new story line of course.

I also think Rain have a lot of potential and has very strong special moves.

This is my thought about this subject

In the Mortal Kombat cartoon. They introduced Rain as a "Keanu Reeves" imposter... And it was a very good look. If Rain came back wearing a more General type costume instead of a ninja one, With his "Keanu Reeves" look and no mask (but still have purple) Then i'd be all for him. He also has a story that they can work with this time around as a General for Queen Sindel and is leading the Edenian army against the Dragon King.

I personally like all the ninjas, Except for Noob Saibot and Chameleon/Khameleon.

(and if you look closly in that picture of ermac punching noob... it looks like noob is a black version of ermac)

I also find it ironic that TemporaryUserName is against the return of "More ninjas" such as Rain. Yet he seems to be abbid for Smoke to return, But to each his own. I just find it ironic.