Well it would have to be an existing charactar, all the new ones are crap. How about someone that hasn't been seen in a long time...KINTARO!!!!

As for how many there should be just one...

I think Kai would make a cool secret fighter. After reading his concept art in MKDA, and learning that he was almost put back into the game, but was cut at the last moment, he had to have atleast something intreresting planned, I'd like to know what that was. So maybe putting him in one of the backgrounds, as first an easter egg, and then as an unlockable.

Reptile. Hopefully he will have split from the DK for MK7, and if he has, I'd like to see him return more powerful than before.
Hidden's have always been more powerful and faster than the regular characters, so in my scenario Reptile returning as a hidden would fit nicely (as well as adding a much needed nostaligic vibe.)
He could even appear before fights (or during - in the background) and give hints as to how to fight him. The clues do not even have to be words, perhaps we could see him crawling out of the background of an arena as an invisible silhouette, from which he emerges only briefly and points to something specific that hints at something to do with accessing him (hopefully these types of ideas are possible on the next gen consoles.)

And when you do whatever it takes to access him (which imo should be VERY hard, so it's an actual challenge, from which you gain a worthwhile reward for achieving something so difficult) there should be some sort of cool cinematic.
A concept I thought up for MKDA would have been, say, achieving a double flawless victory and performing a fatality (just an example, but the goal could be anything) on MAX difficulty on the Portal Stage. If you did this, the portal opens and transports you to one of the conceptualized stages (the ones thought up, but not created from the Krypt) where you would fight Blaze - who would be extremely hard to beat. If you beat the hidden fighter, then and only then should they be unlocked for you to use.
So if that idea was used for MK7, you would be transported via the Portal, to fight an incredibly difficult Reptile (sped up, awesomely overpowered), and if you beat him, then he would become unlockable (and possibly be able to be used online???).

So, um... yeah, Reptile

u know u made 2 of the same thread.

Awesome ideas as ever!

And I would pick Reptile also. I want him back along with Shang, Quan, Sonya, Kung Lao and Kitana!

Interesting topic here. I think it's hard to say. Most of the characters I want back I'd much rather have as normal characters, or unlockables like the recent games had.

I think they should expand on the idea they did in MKDA, what they were supposed to do in MKD. Have a couple of characters not part of the main cast, not unlocked like the others, but discovered in other ways. These characters, of course, should be better than Blaze and Mokrap were (Blaze had potential, but was a rush job, what was Mokrap all about?). They should have complete, unique styles, movelists and finishers.

Maybe unlockable after a certain number of wins, winning in certain conditions, or time release, I'm not too sure yet. Like they did in MK1, I think having random clues as to their presence in the game would be a great touch. As an example of what I'd like to see done this way, here's my ideas. I'm gonna go with four fighters for this.

1- Kurtis Stryker. I know most people hate him, but I think he's great. He hasn't been seen in a 3D MK game and is very unique in not using magic fire or whatever. Give him a real SWAT uniform, and some police/military fighting styles, and I think he could really be a really cool comeback.

2- Sareena. For a character who's yet to appear in a proper MK game, she's actually very well developed. Many of us would like to see the sexy demon make her debut in the main series, this would be a good way for her to do so. Given unique moves, she could be a real keeper in the series.

3- A character we've seen as an unused concept. There's quite a few cool designs that didn't make the final games. How they dropped some of these in favour of worthless turds like Kobra is beyond me. Personally I'd really like to see Elder Demon Baphomet finally make his MKD debut.

4- An all new character. Obvious really, someone we've not seen before in the series. This one should probably be the most well hidden, with only small clues to his/her existence until they're found. No clones of existing characters, or Mokrap style jokes.

i would like one or two hidden fighters who are bosses and are much stronger than the other characters as well as about 10 other hidden fighters that you unlock berfore you unlock the boss and sub-boss.

it was a shame that onaga and moloch were not playable at all, i don't
want them to be immedily playable but i would like to unlock a big strong unthrowable, boss as a prize for completing the game with all the characters.

like: motaro, onaga ,moloch ,kahn or some new boss that;s even cooler

They should have scorpions brother! HE could be a strong fighter here to help