Immersion into the Duchy
Characters are not created and do not come into being within a vacuum, each should have a past, including relationships and their reason's for existing. A backstory should not only give background for the characters, but introduce a slew of potential storylines for use in a campaign.
Listed below are the elements that I would be most interested in understanding about each character class. You may, of course, expand and explain well beyond anything listed, but they are what I can think of to better integrate your character the world I'm trying to develop.
Of course, there is a fine balance between providing pertinent information for use within a campaign and spending hours upon hours writing up something that may eventually fall by the wayside, either because you are not interested in playing that character (you will eventually have to choose between one of the two) or your DM overlooks some elements in favour of something else.
Using the "Creating a _____" sections and answering most of those questions presented would be a great place to start. While backgrounds provide a useful and useable means to start a character's development, so much more needs to be rounded out. Family and relationships are always important. If a military character, rank and people worth caring about? How did you get there and why? Keeping in mind the level and age of your character.
To be added should be relationships and, as it is an immersive effort, how the character functioned before this introductory adventure. A hint from the efforts of previous storytellers and authors: characters created from the extremes may sound the coolest, but are the toughest to have their stories continued. A boy raised by wolves or a street urchin with no idea who her parents are difficult to continue without falling into established tropes or relying too much on the DM or player to fill in the blanks as to what comes next. This makes it double tough as the game is not intended to be developed around one character by several.
Players should have a ton to say about how their characters appear, are motivated and present themselves in-game. This starts, in my opinion, by them contributing to the world as well. Backgrounds should include the important aspects of the class, such as; mentor, trainer, chief, monastic rules, tenets of the faith, guild policies. Each invited has the imagination to develop enough, but know when it becomes beyond what a character may be privy to and, therefore, risk stomping on the DM's master plan.
Finally, having some idea of what your character may be aiming for would be great. In a meta-gaming state of mind, where does your player hope to take that character and how would that show in-game. Again, tread carefully as this is the most malleable of all the aforementioned portions of your character's development. That said, it may be prudent for a player that would like to run a barbarian from a tribe whose religion is based on animism to have some thought in choosing the Path of the Totem for their character, practice certain portions of each until eventually choosing which particular totem come that level.
I would like to imagine the withdrawal of the meta-gaming, power building of characters that often crops up in games. Where would a halfling bard who made the early stages of his career pickpocketing around his small town learn Infernal as a language slot. A successful leader with a charisma of 8 isn't very likely either. Power building from the outset, using stats, feats and traits to develop an awesome character is not in the spirit of an immersive game and one that strives for characters to ebb and flow with in-game events.
Examples of specifics for particular classes come below…..
Where are you from? There are tribes in the mountains in the area the campaign will start, a mine that the goblins are running through, plenty of forest.
What are you harnessing onto to fuel your rage; animal spirits, your ancestors, war god(s)? Should be based on above, in that are you a homesteader, tribesperson, etc.
Anticipated class path and how are you going to deem yourself worthy to become a protector of your people, successful resister to the pressures of the land, defender from the goblins.
Particular talents and desired use for them; eg. Instruments, drama, oration, combination. Instructor's name and relationships.
Motivations toward bardship.
First gig? Have you played one? Description of how it went.
Anticipated college and what you would show to apply.
How did you begin practicing the faith? What is involved and expected? Who is in charge in your area? What is the personality and development of the faith. Relationships within this place.
Description of the location, church, retreat you belonged to and set out from.
How did you come to be an adventuring cleric?