I'm not sure I understand what you mean. If you're asking whether dragoon is going to play just like monk because everyone is going to be choosing the same supplemental abilities from other classes, that is very unlikely. If, on the other hand, you're asking whether every dragoon is going to select the same supplemental abilities as other dragoons, then this is very likely.

Given that the game mechanics are being overhauled all the way down to how stats function, it's going to be a while before the community figures out optimum builds. That said, as in any MMORPG, the community will eventually settle on optimum builds for each job, and people will probably be considered gimp if they don't adhere to them. That's just a consequence of cooperative gameplay from MMORPGs to online multiplayer in a. Certain play styles or builds will be established as the most effective, and the community will adopt them and then pressure others to follow them.

Given that jobs have limited options to begin with, I wouldn't expect much (if any) variation. Part of the reason that every BLM used chameleon and quelling strike in 1.x was because two abilities that mitigate enmity are indispensable to a job that tends to deal a lot of damage and thus draw aggro. The other reason was because damn near every other ability they could take from archer involved a bow and arrow and was therefore useless to a job without one.

If you're playing as the base class without your job active, you'll see greater variation depending on the task at hand. However, this is only going to happen when you're flyin' solo. Group play will lean heavily toward cookie-cutter builds optimized for a specific purpose.