The biggest assumption for the design is:
Greenfield is an economically sustainable, service-oriented ecosystem.
By "self-sustained", it means that the service providers and corresponding service consumers of Greenfield are rational; they complement each other. The providers and the blockchain validators will get paid a fair amount for the service they supply, while the users are willing to pay for the service they use.
By "service-oriented", it means that the value was created in Greenfield by providing service to the users of the ecosystem. It doesn't have built-in value by itself.
The implicit assumption underlying these two traits is that the majority of the providers and blockchain validators are reasonable entities and individuals. They will not do evil given the profit they earn is larger than the fortune they can plunder.
This is a self-justifiable trust for the whole ecosystem to exist: if a substantial percentage of providers and blockchain validators do evil and the ecosystem cannot heal itself by ruling out these malicious players, the whole ecosystem will not be used and have no value to existing. If that happens, nobody wins even for a short time.
With this implicit built-in trust assumed, many designs are simplified as described in the below sections.
Another assumption is that both the service provider and consumer sides would expect the actual "service contracts" between the two to allow limited liability and provide exit options, even if the contracts are carried out mostly by code. In consumers' interest, they do not want to pay a large size of fund upfront and would like to choose a better provider within the ecosystems or even outside whenever they want. On the other side, in the service providers' interest, they do not want to become a data wasteyard or help circulate any content against their own principles.
The payment, data availability check, and a few other key features are designed based on this assumption.
The last big assumption is that data has value and users will want to extract this value with smart contract automation, privacy, and transparency. This results in the considerable design for cross-chain between BNB Greenfield and BNB Smart Chain (BSC). This should be the most important assumption and hopefully close to the truth.