Making money is tricky overall, because your own passion for an idea might not return much.
That’s why sites like Steamspy can be invaluable for spotting trends or gauging interest etc.
Everyone hopes to make some $, but its best to make games either for money or for passion.
That way if the 2 combine you’re laughing, but since that’s never a given, treat it like any job!
Personality / your own game taste may actually dictate which game type will be easiest for you.
Plus one game type isn’t any easier than another if you plan on highly custom game mechanics.
Best to hunt for game kits in community tools / buy from marketplace to make a good head start.
Long hours if you can actually do it, will serve you well as games are all about total commitment!
There’s always ‘flappy birds’ like quick easy successes, but that’s stuff of legend / lottery tickets.
Drawing from UDK as its been around longer: 2-5 years dev time is not uncommon for FPS / RPG.