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Since singleplayer, multiplayer and MMO are all inter-connecting and blending together. There is little to tell the differences between some of them. Games considered multiplayer, are now called MMOs...MMOs now have many singleplayer features.
Singleplayer games (like Skyrim) haven't taken the approach of MMOs however. There is no cash shop, you can't play Skyrim for free (legally) and there is no score board or anything.
My idea is this...take a singleplayer game, turn it free to play (it works really well for MMOs)...and then charge for extra content. I'm thinking every quest should cost 1-2 dollars...and the major quest chains (like if something similar to the Dark Brotherhood), it should cost 5-10 dollars. Of course still having regular expansions and DLCs. Also have unlocks, like certain items are locked (more so, the rare ones) and you have to pay to unlock them. But leave the main story line, the basic items and basic crafting, all free for the player.
I think this approach would work really well, and make the company doing this, a lot of money. Everyone dives into a free to play MMO, and spends more than a normal pay to play MMO...and they carry all the free to players. MMOs that went free to play or freemium, from pay to play...have seen a HUGE boost of sales, income and population. And I think it could help singleplayer games a lot. Not just Skyrim, but racing games, FPS games...other RPG games...I think it would do really well.
If all someone wants to do is go through the main story, and not worry about sidequests or leaderboards...he can do just that and not pay a cent. However, I imagine many would want to do all the side missions, unlock the rare items and buy items, exp bonuses and everything. The company would make more than enough, to offset the free players.
My Skyrim, Fallout 4, Starbound and WoW + other game mods at MODDB: