The one quibble that I have with George's post (which I think is great) is the long focus on rank and how credible it is or should be in certain situations.
Personally, I don't recognize anybody's rank - I question them all, whether they come from Mary, Saotome or Morihei himself. Other than that, we meet, we train, and I start to form on opinion.
The whole rank thing, which was started only to meet requirements of the pre-war Dai-Nihon Butokukai, was IMO a big mistake - especially the attempts to implement it organizationally.
Take a look at any professional organization in the United States - one thing they have in common is that they have no ranks. There are certain minimum requirements and certifications (much more similar to the certificate system in Koryu than to the Dan-I system), but other than that you don't have, for example, a 5th Dan Optometrist vs a 7 Dan Optometrist.
I would say - get rid of all the ranks, except as a personal and optional thing between a teacher and a student, and create an association based on providing real benefits and resources to its members, not on mail order certificates conferring imaginary levels of proficiency.
I think moving to a koryu-style system of licenses wouldn't work. It would wipe Aikido out, leaving very small groups that would not be as interested in coming together and training. There would be much less quality control.
Essentially, what you would be doing would be removing kyu ranks and most of the dan ranks. When you were fortunate enough to get an interested prospective new student, the first recognition she would receive would be a "you are officially a beginner" certificate after some years of training. Then after that, another several years of hard training until a "you can do this stuff well enough for us to tell you a little bit about what you are actually doing" certificate. Then many more years until a "you can do this in public" license, and then years after that if you can endure the disillusion and outrage, you MAY get a "you now officially own what you have been doing and can go out in the world and teach it" license.
There is no assurance at all of quality in this setup. You could still have instructors giving ranks out for whatever reason. But worse, this kind of system doesn't seem to have much to offer someone who is not very serious. You are not going to get people who are looking for something that is good general exercise with a kinda cool philosophy, or even the people who really, really love it but just cannot keep rolling that boulder up the mountain week after week.
So you aren't going to make rent, bottom line.
Now, I think a very personal, hands-on relationship between student and instructor is a better way to transmit skills than the "lecture" style of most mainstream Aikido dojos, where the instructor manifests some awesome stuff and then everybody pairs off and works on it to the best of their understanding and ability. But the lecture style accommodates a wider range of student. And furthermore, the lecture style is traditional - Osensei and Takeda both had personal students but they also taught seminars. Yoshinkan was designed from the ground up to be taught to large groups.
if you shrink the classes and select only the most focused students, aside from shrinking the global Aikido population dramatically, I think you will also make the groups less interested in training with each other for a variety of reasons. You would be less interested in attending a seminar over the weekend with a visiting shihan, because what are you going to do afterwards? Come back to your regular class, where you are being personally molded by your own sensei, and try out the new stuff you learned at the seminar? Also, since you've removed the rent base from the art, you are probably only training in someone's garage on weekends anyways. So you'd have to miss your regular class. I have been training with two small koryu groups on weekends and it is just too important for me to attend every single class i can, to skip off and attend all of the wonderful Aikido seminars that are hosted around my area. Its not just that I have such a limited opportunity to practice these things regularly, its that I have an obligation to my instructors to be there so that they aren't wasting their time. I think deepening the relationship between student and teacher with a koryu-style system of training and rank will raise these issues of obligation.
Several koryu of which I am aware seem to have adopted kyu/dan systems anyways, for the purpose of fostering larger communities. The iaido and jodo that is practiced under the auspices of the kendo federation, for example, and also Hontai Yoshin ryu. So that tells me that when the decision is made to "go global" the more impersonal, organizationally-oriented belt rank system is a comfortable fit.
The internationally-recognized belt ranks might be one of the best services the Aikikai can provide. It provides a sense of commonality to practitioners around the world. Even if quality fluctuates, if you've got people from different parts of the world willing to train with each other, there's the chance that the quality gaps can close themselves as people meet and train with people who are better than they are.