Planning-on-Paper Is something I find myself doing often. In fact, this is the purpose I use Design Docs for and I create one for every project/subproject. Its good to see this in Development Threads because its part of the development process.
Stats definitely require a plan, because they can be various datatypes, numerical tickers or timers, scalar or composite, nested / linked to other Stats, hard coded or soft coded, volatile or non-volatile, replicated or non-replicated.
My inspiration for designing Stat Systems is based on the concept of simple key:value pairs which can be easy to map to other data, serialize, and parse. I constantly eyeball using UE’s Gameplay Ability System but keep getting the feeling I need to roll my own Stat System. This is where i find the difficulty.