Stormdragon
You sound like you are asking simply what goes in to developing a form. Heresy of Heresies, how dare you!
I've never created a thesis form though I always thought it would be a fun thing to do. I recently created my own version of Hangetsu.
To start I decided on what I wanted the kata to teach which in this case was based on my understanding of Hangetsu. I decided to catologue the linnear close quarter fighting concepts I'd learned. Then I stole Hangetsu's embusen and changed the basic movements to movements that fit what I wanted to see in the kata but initially keeping to the general types of movement and keeping signiture movements in tact.
To the places that repeat themselves in this kata I assigned combinations that either had multiple applications or that were some combination of being: very useful combatively/key technique-wise or that were good exercise, I then added a few movements here and there that changed the embusen slightly, added an extra sequence that I thought was important and changed the breathing to include some hard qigong in the first half of the form and controlled combative breathing in the latter half.
This was not done for anything other than my own entertainment, I saw holes in Hangetsu that were not filled by any of the other versions I looked at and thought it would be interesting to devise my own. After all there are more versions of this kata than almost any other, so what would one more hurt. I'm quite pleased with the result.
I suppose from my perspective considering what you want the kata to show and being consistent to that aim is the most important factor in creating a new form and it should guide everything that is included or removed. You must be clear on what you want the movements to achieve. If movements repeat, why are they repeating, why do that movement here and not there in the form?
One thing to note is that I was not concerned with everything flowing together into a continuous bunkai sequence as I see Hangetsu as a series of short set pieces, each one teaches an important principle and each principle occurs as needed in actual combat, often combining with two or three other principles from other parts of the kata. You could do the same or you could envision a fight against a skilled opponent and display your side of that fight in a continuously applicable and easily deconstructed sequence. Those are just two of many extremes.
Also a kata can just be a training exercise, theres no reason not to just combine your favourite combat based solo exercises and call it a kata or to intersperse the fighting techniques with exercises that make the whole kata a challenge to perform.
Is this kata a fighting system in its self? Is it part of a system or some key self defence concepts strung together? Is it a way of practicing your favourite moves? Another easy tool would be to use the HAPV model i.e. consider the most common (Habitual) Acts of Physical Violence a person might encounter (hook punches, verticle swing with a glass bottle etc etc) and devise some effective defences against them.
Perform 75 Gedan Barrai's in a row and see if anyone gets it!
Remember to consider performance. Is the kata explosive from the start, does it contain slow controlled movements? if so why? there is another thread about the why of tempo changes and personally (just my opinion) I don't think any of the answers on that thread are what was intended by the kata builders, but then who knows? What do tempo changes mean to you? In my Hangetsu the slow movements are for the qigong and for practicing balance and muscle control, the fast movements are to practice speed and the shift from one to the other is placed to show the need for and develop explosiveness when attacking. Do you kiai? Are there Kiai points in your kata? If so why are they where they are?
They are for me the most important points that came to me.
(My apologies for the dis-jointed nature of my writing).
P.S
What is it about the forms and applications Forum that has everyone posting reasons to avoid thinking about the question instead of just answering it?