1-Is it appropriate to start a new martial art? Please answer with a yes, no or maybe and give reasons why. Also, if you feel that certain parameters must be required then mention them as well.
Maybe. Depends on who's starting it and why.
Everyone ends up creating their own
style. If they build a curriculum around their style, then they've created their own
system.
A new
art ... by my definition, that's pretty difficult to do. Really, there's only one
art -- martial art.

The term
art in standard MA vernacular usually refers to a set of martial arts systems from a specific country, culture, or region. Since most of the world is already covered with
arts, then whatever system you create will generally be considered as part of:
(a) the art(s) of the region in which you reside or
(b) the art(s) that you used to create it
So, for instance,
Eskrido is a system created by GM Cacoy Canete. He was among the top Eskrimadores in Doce Pares, a 3rd Dan in Kodokan Judo, and a 2nd Dan in Aikido (I belive those are the right ranks). He took the locks and throws from Aikido and Judo and blended them into his Eskrima and called it "Eskrido." It is considered a Filipino martial art because he is a Filipino, lived in the Philippines when he created it, and its primary foundation is Eskrima.
Shen Chuan is an American system because it was developed by an American in East Texas, Joe Lansdale. He had studied and had rank in quite a few different martial arts but his primary influences were American Kenpo, Hapkido, and Daito Ryu Aikijujitsu and he'd also been influenced by some Tai Chi, Chin Na, and other Chinese systems. He didn't set out to create a new system. He was teaching a group of guys and just doing it "off the cuff." One day, one of his students, Eugene Frizzell, said, "Man, Joe. This is really good stuff. You need to make a curriculum and start teaching it publicly." So he did. He used the Chinese name to give credit to the influence the Chinese martial arts have had on MA in general and, specifically, on his own development. But it is very much an American martial art.
If, however, someone is intending to create a system and they take some stuff from here and there and throw it into a salad, they don't have a
system. They've got a bunch of disconnected elements. This is why so many new systems tend to suck.
When the creator develops a cohesive whole that can be effectively taught, then it's a good system.
2-Is it appropriate to make a derivative or variation of an existing martial art such as "Combat TaeKwonDo" (I don't know if this exists or not but is simply an example). Again, please answer with a yes, no or maybe and reasons why.
I think my answer to #1 applies to this one as well. And, FYI, I think "Combat TaeKwonDo" is an existing art. Or, at least, I've heard the term used by some people.
3-Is it appropriate to start a reality based self-defense system? If you do not believe that they exist and are simply martial arts in another name then please say so, otherwise please answer with yes, no or maybe and of course reasons why.
I believe it's all martial arts. I think there is a finite (though large) number of ways that the human body can move, be moved, or be affected. Martial arts limits that number further by restricting it to methods that cause or prevent injury. This creates a pool of concepts and principles that are universal. All systems of martial arts draw from this same pool. The only thing that makes each system unique is its approach to the pool and which area(s) it prefers to drink from.
Different systems are geared toward different environments. But, in my estimation, any system which draws from these universal concepts and principles will be valid and applicable. Whether a specific systems "flavor" appeals to a given individual is an entirely different subject.
The bottom line is that martial arts training just improves our odds. The person who is most capable with his/her tools and can adapt them to the environment/situation will have the upper hand. This is true whether it's in a ring, on the street, or anywhere.
Further (and straying a little from the topic), I'd say that this concept can be extended into any kind of environment from verbal confrontations to office politics.
Learn the tools. Delve into the underlying concepts and principles (it's not enough to have a tool box; you have to understand
why the tools work and how best to make them work for you). Then apply that knowledge to the situation and environment at hand.
A lot of people get caught up at the technical level. They put together a tool box (or get a pre-packaged one) and they put it on a shelf and never explore the concepts and principles of what really makes those tools functional. When people like this create a system, they end up with a hodgepodge Frankenstein's Monster that doesn't hold together in the long run.
Personally, I have no problem with people creating their own systems. I mean, think about it, at one point or another
every system was a new creation.
Mike