What is the thematic meaning of the prohibition of chametz on Pesach? I mean, besides the fact that it's puffy and we want to avoid an over-inflated self image.
With perhaps one exception, matzah is mentioned in Tanach in only two contexts: the Pesach holiday, and various Temple offerings which must be matzah rather than chametz. Perhaps then, the meaning of chametz is related to some concept which appears in the Temple service.
The most relevant such concept is a law of purity and impurity known as "hachshara". Basically, food which touches an impure object does not itself become impure. But if the food comes in contact with water (or certain other liquids), the food is "made susceptible" ("muchshar") to impurity. From then on, if the food touches an impure object, it DOES become impure.
This is similar to chametz, which also is transformed to an undesirable state by contact with water.
Of course, the criteria for chametz and hachshara differ somewhat. To take one example, flour which is mixed with water immediately becomes susceptible to impurity, while it does not become chametz for at least 18 minutes. But the basic idea, that food must be kept away from water to ensure that it remains acceptable, is of course common to both.
The symbolism may be that in order to separate ourselves from the impurity of Egypt, we have to separate ourselves from the possibility of impurity in our food, by keeping the grain away from water until the last stage of the baking process.
At the same time, you cannot make bread without using water at some point. If you can get through the dangerous potential-impurity period and end up eating food which has remained pure, that is the best situation of all. Perhaps that is what we aspire to on Shavuot, when in contrast to Pesach we not only eat chametz, but are required to offer a Temple sacrifice made from chametz.
And the Omer, which is a critical and potentially dangerous period in the agricultural year (this seems to have been its original significance), represents the period (after "hachsharah" and before eating) in which we must carefully watch our food to ensure that it remains pure.