Whether you join as a Canon character in the Next Generation or an OC character in the Marauder's Era, what happens in those Era's is entierly up to you. Of course, there will be a general plot going around for each of the Era's but this is generally to just get you roleplay with your favourite characters in different scenarios.
I will, however, be making sure that you don't take things too far. Do try and keep it to canon a little bit, for example, if you apply for Harry Potter, who will be in his Fifth Year, do not change anything that happened before Fifth Year. Anything after that is up to you. Whether you have him turn from the light and go neutral it's up to you but before that point keep it to canon.