You are flying around on a nice VFR day and the engine suddenly starts to run roughly - what would you do?
First apply the POH actions for rough running engine on that type.
To answer further, need to know
What altitude
What is the fuel state
What track I am making good?
What is the wind direction/speed
What weather conditions (nice VFR day is pretty vague - if its -20 degrees it may skew the decision)
Over sea or land
If sea, are there ships nearby
What is the sea state
if land, what type of terrain
How close is nearest populated area
How near is nearest usable airfield?
Any physical signs of why engine rough - e.g. oil or smoke?
Depending on the answers to the above, I might consider a number of decisions