Making a game of such huge scale (whether 2D or 3D) in any game engine would be too much work for one person, I would have thought. I'm no expert.. but from the exposure I do have, a lot more work goes into even the most basic functions of a game than I was aware as a gamer. It's been a real eye-opener. Even in a semi-WYSIWYG development platform like Stencyl or GMS2, nothing grows respect for game devs like trying to develop a game. Of course there's a lot of game logic you'll only have to pull your hair out over once and then once you have it working you can export it for use in other projects, but still, even with a basic single player shmup a huge amount of time goes into the game's logic without even considering time spent on art, sfx and music. It takes a team comprised of concept designers, coders, visual artists, sound designers, musicians, testers etc. several years to create an MMO. For one person I'd imagine it would take a life time, even scaled down as you mentioned. There is the old expression "Never say never"... but there's also the expression "Glutton for punishment".
