Well guess what: most successful f2p games are all clones of existing games. Or at least close adaptions.
Lets check some out.
Clash of Clans (iOS)
The best monetizing f2p game from the west does 2 million US$ a day, but the team did a close to identical game called Galaxy Life in their former company Digital Chocolate, which again is an adaption of a Facebook Game called Edgeworld by Kabam.
By the same developer Supercell this is basically a Farmville variant. A good one yes, but still a lot of mechanics were taken from that old Facebook game by Zynga (which in turn cloned it from an Asian game)
League of Legends
Yes, even they adapted the grandfather of Moba, a Mod for Warcraft III called Dota. And yes, Dota2 is the sequel. Of course the setting and graphics changed, but the fundamental principle of the game remained the same (and yes, I know the guy is one of the original Dota mod creators)
A match three game with RPG elements and monster fusion meets Pokemon. The most successful mobile game from Asia even dwarfing Clash of Clans in revenue. Variations were available before (like Puzzle Quest) but not the mix like they did. Clones are appearing daily.
Candy Crush Saga
Well take Bejeweled, attach a meta game and progress, voila you have a game which does 30m a month. Many tried but King.com Saga recipe seems to work for casual games. Only after Candy Crush moved to iOS it bat their former revenue king Bubble Witch Saga which doesn't work on mobile as well due to interface issues. Clones have been tried, the more successful ones are even from the same publisher King.com. None can reach the success (and if you wonder there is a reason for it).
World of Tanks
Counterstrike with tanks. Nothing special right? Attach a progression system and monetization and voilá you have a hit. I might be too simplistic but under the hood WoT is nothing else.
So what did those games above do to actually be that successful? The numbers are extreme. Millions of users, millions of revenue even daily. The above games dominate the f2p online space. No wonder companies try to clone those games - some even 1:1- all those images are from a different Clash of Clans clone.
It seems there is enough potential in old successful game loops that you simply take these, attach a meta game with progression and monetization strategy and you might have a million seller. It seemed to work for the companies above right?
What really happens is that f2p is still young and exploring new areas, so expect more mature games (or more core as you would say) to expand into f2p space soon, so that the two leaders LoL and WoT can be challenged.
Also expect any game loop you know to be misused as a f2p game at some point. Some already have been tested like Black Jack, Golf, Poker, Asteroids, Mahjong etc. It is a formula and simple (core gamne loop + Meta game & progression) and will be exploited as such. It is so simple that I wonder why the huge wave of these games hasn't arrived yet.