That sounds great! I just hope the dev time won't be overwhelming.
That said, I have some ideas - not recommendations, just systems I've seen in other media that could be inspirational.
Skill System:
Skills and Arts, wherein Arts are active skills (actions).
A rank system, each rank has multiple levels in a rank. Makes mastery feel more achievable and rewarding than only 10 levels total or a whole 100 levels.
Sacrificing levels in a skill/art to gain access to a new skill/art.
Jobs:
Special jobs unlocked by failing to get into a job or failing at the job.
Relationships:
A gossip system. Not only is the economy interconnected, but relationships as well. If you do something to someone, other people know about it. This gossip is weaker the further you get from the incident, but actions with larger impact spread further and are not easily forgotten. This goes both ways - npcs know about other npcs and events not directly related to you, and the usefulness of this info depends on your playthrough. You can maybe find what someone is going to sell next week this way.
Whether this should be done also depends on its effect on game size and ram.
Inventory:
Tile based, weight based, etc inventory. Just something limiting but not annoying. Keep it viable but not bag of holding type of space.
Other:
Things shouldn't be too easy or difficult. You should always be able to steal but should always have a chance of getting caught. Debuffs should be debilitating but not gamebreaking. Etc.
Also, F A S H I O N :D
I look forward to more!