They all come surging back when the designing starts, so that none is overlooked. Thinking of implications brings up the topic of logic. When we look at how the system will function, it is natural to explore design alternatives. And that is a fair thing to do. Even more important, however, is recording the thinking that went into the decision so that it can be easily reversed at a later stage, when new technology is discovered.
Negation is the last important part of the logical process.
The statement above says that the required features imply incompatible designs. That means one of the design ideas has to be dropped. In consequence, unless some compatible design can be found, one of the features must be dropped. This is fairly complex reasoning, but it is typical of the design process. For example, previously I had argued that the system's documents should be stored in plain text files, because of the advantages that attend that strategy.