My answer, as you could probably guess, is to make it all random, including dream content, and decide on importance later.
My first and I think best instinct is just to have a 1 in 6 chance to remember a dream after every sleep period during an adventure. I believe the way I would handle it is: call for all players of sleeping characters to roll a d6 when the PCs wake up. Roll a d6; any player who rolls a match had a dream. For each, roll 2d20 on the six-attribute dice map; the location of the low roll indicates a general setting of the dream, while the location of the high roll indicates the general action of the dream:
- Strength: locations and actions associated with fighters, like battlefields and duels
- Dexterity: locations and actions associated with thieves, like alleyways and hiding or climbing
- Intelligence: locations and actions associated with magic-users, like libraries and rituals
- Wisdom: locations and actions associated with clerics, like temples and chanting, prayer, or sacrifice
- Constitution: locations and actions associated with bodily needs (inns and taverns, outhouses)
- Charisma: locations and actions associated with social behavior (courts, parties, festivals)
If players say they try to remember more of the dream, it takes 1 turn of mulling it over. Roll a number of d20s equal to Int/6 on the dicemap to add more information from the dream. This can only be done once. If a player attempts to use magic in a clever way to recall even more details, roll 1d20 per spell level on the table. Other methods to forcibly remember more details have to be judged on a case-by-case basis.
If any dream element matches something in the current adventure or dungeon, the GM rolls a d6: on a 1, the dream is relevant and not just fantasy. If a player comes up with an interesting interpretation of the dream, the GM rolls a d6 with the same odds to see if the player's interpretation is true or becomes true. Let it all develop in play!