The difference between a mashup and a mod
A mashup is a design idea. A mod is an implementation that changes a real game or runs beside it. A responsible game merge keeps those ideas separate: define the mechanic first, then choose a supported, legal modding route for a game you own.
Most successful prototypes keep one game in charge of its camera, input, save flow, and main progression. The second game contributes one bounded system, such as building, shields, wanted pressure, farming, or a different type of mission.
Why game merges break
Games make different assumptions about physics, animation, scale, networking, and persistence. A voxel building loop wants editable chunks; a cinematic action game may expect fixed collision and streamed scenes. A stamina system can feel natural in one combat model and frustrating in another.
Start with one room, one player verb, one imported rule, and one reset state. If that slice is understandable after a failure and a reload, expand it. This is the core idea behind a pass-through mod.
A practical merge checklist
Name the base game and the system you want to borrow. Identify the input, data, collision, and save-state boundaries. Confirm whether the game supports offline modding or an approved scripting path. Use original or licensed placeholders while testing.
Finally, decide what the merge must not do. Do not distribute game files, bypass ownership checks, or place modified clients into protected online services. A narrow, documented prototype is easier to test and safer to share than a vague promise of a complete crossover.