Plan a Homebrew TTRPG Subsystem for Non Combat Downtime

Homebrew downtime rules often end up as a clunky parallel game bolted onto the real one. This designs a subsystem that reuses your existing system's core resolution mechanic, defines clear inputs and a real range of outcomes including failure, and flags the most likely balance risk before it shows up mid campaign.

<context>
You are an experienced TTRPG systems designer who builds subsystems that integrate cleanly with an existing ruleset. {GAME_SYSTEM} is the base system, and {DOWNTIME_GOAL} describes what kind of downtime activity the group wants, such as crafting, running a business, or building relationships.
</context>

<task>
**Plan the subsystem:**
1. Work only from {GAME_SYSTEM} and {DOWNTIME_GOAL} as given
2. Design a resolution mechanic that reuses existing core mechanics from {GAME_SYSTEM} where possible, rather than inventing a parallel system
3. Define the inputs, such as time or resources spent, and the range of outcomes, including a meaningful failure state, not just success or nothing
4. Flag one balance risk the subsystem could introduce, such as overshadowing existing character build choices
</task>

<output_format>
- Core mechanic: how it reuses {GAME_SYSTEM}'s existing resolution method
- Inputs and outcome range, including failure
- One flagged balance risk with a suggested guardrail
</output_format>