Have you ever felt that something was missing in the Reiki system we have today?
If you mean "Western" Reiki, I have always felt that there is too much flash and not enough fire.
If you mean Reiki at its most basic level (mainly the underlying philosophy), no

Once the philosophy is understood, the need for other systems of "palm healing" melts away. Of course, other disciplines like acupuncture, herbal medicine or Tui Na are still useful.
Usui Reiki Ryoho was specifically developed to make enlightenment attainable within a lifetime. The foundation of the system was laid over many years of contemplation and fervent study. I have to laugh at anyone who feels that they can more or less instantaneously "channel" something superior. No spirit guide or ascended master
in his right mind would ever help you avoid having to work hard by giving you or anyone else a more powerful symbol or a secret key to enlightenment simply because you asked nicely. It would not be in your best interest. Our society, which puts so much emphasis on instant gratification, is at fault here.
An important test of a Reiki practitioner's skill is to be able to let go of control and manipulation and let the healing occur, with love and compassion as one's only guide. By this, I mean to say that the act of healing, and the purity of one's intent, is more important than having precise control over the method. This is the lesson of Hon Sha Ze Sho Nen -- "Correct mindfulness is the essence of being."
The desire to maintain control is a fact of human nature, but it is not conducive to either profound healing or transcendental awareness. Yet, it is common for people to focus on minutiae, because it makes us feel good to do so. I didn't want to resort to this, but consider this Bruce Lee quote (hehe):
"It's like a finger pointing a way to the moon: don't concentrate on the finger, or you'll miss all the heavenly glory."
We as humans are always searching for the next big thing, the most powerful energy, the best symbol to unlock the ultimate healing power, but there is no substitute for practice.
Perhaps the best thing that someone can do to this end is to embody the five precepts:
Just for today, do not anger.
Just for today, do not worry.
Just for today, be humble.
Just for today, do your work honestly.
Just for today, be kind to every living thing.
There are a number of reasons why practitioners may not be content with Usui's system, perhaps the most prevalent being that the Reiki that is taught most widely today has little in common with what is known of what Usui taught during his lifetime. Early Western teachers adapted the teachings to be more in line with their world-view, after some amount of misinterpretation due to cultural differences.
Regardless, there are no shortcuts. If you want to find the truth, you have to be prepared to invest time and energy toward that pursuit.
There has always been some conjecture that the woman who brought Reiki into this country, Mrs. Takata, was not given all the steps.
That is a popular myth, yes. The truth, of course, is that it became an issue when certain restless souls began searching for novelty. These people will never be fully content with what they have, and so they are caught in a continuous cycle.
For more information on what Hawayo Takata actually taught, see
this article. This particular site provides quite a bit of evidence (in the form of diary entries, etc.) to suggest that we have not given Takata the credit she deserves.
Did you ever feel as though there was something more to Reiki than what you were taught?
Yes. In fact, I was never taught anything -- I only received an attunement. It has taken me five years to reach this point, with the bulk of my major discoveries occurring in the last few months. The point is, I never gave up on Usui Reiki Ryoho, even when I might have wanted to

Today's Reiki enthusiasts want a more active role in healing themselves and others.
What you're describing is precisely why so many modern Reiki practitioners will fail to reach enlightenment.
If someone
receiving Reiki treatment wants more of a role in facilitating his/her own healing, that person should learn Reiki. However, when you think about it, such an individual has already taken that responsibility by simply accepting Reiki. Someone receiving Reiki can also speed up the healing process by beginning to eat healthier foods, which again is his/her responsibility to continue doing.
However, it's time to look beyond Reiki's traditional structure and see if something may be missing.
Who says so? I still haven't seen any compelling evidence for this. You just need to look harder at what is in front of you, IMHO.
... Reiki can provide a more complete, lasting and holistic approach to well-being.
Usui Reiki is more complete than you seem to realize.
In my opinion, if the benefit you have received as a practitioner has not lasted, it's because you are still searching for something that "just works." That is fine, but remember: no shortcuts

Usui Reiki
is holistic in its approach:
relating to or concerned with wholes or with complete systems rather than with the analysis of, treatment of, or dissection into parts <holistic medicine attempts to treat both the mind and the body*>.
Conversely, you appear to advocate a more controlled, analytical method.
*The original Reiki precepts even end with the phrase "Improve mind and body."
I agree with you on one point: it is important to accept responsibility for our [physical, emotional, mental & spiritual] problems. Only then can we release them

This is not an issue of blame, however, but self-acceptance and love. This is something that I would teach before "Level III," meaning I still don't see the need for a fourth, nor will I ever.
Personally, I think
oOwabi-sabiOo has the right idea.
I should also mention that I'm not trying to deny the existence of various forms of spiritual beings. However, I maintain that the connection between us is not always reliable -- making it a bit like a game of "broken telephone." I don't often try to receive guidance in this way, simply because the human brain loves to fill in the blanks.
Would you be offended if I asked to read your doctorate thesis?
Reiki ni reiHarry