One of the things I've noticed that can affect the long term viability of F2P/P2P games is the ability to exchange goods or items.
I started out in the Mobage world with Transformers: Legends. It wasn't bad and it captured a lot of the G1 era nostalgia for me. However, one of the issues I noticed was the limited availability of cards/characters. For example, if there was a week long event, some of the available premiums could appear in packs. To maximize a unit, you had to acquire both halves (robot and alternate forms.) If you pulled an ultra rare character, but only one form, you were pretty much in a bind. See, in that game, there is no trading feature. This means that you have a limited inventory of cards/characters and unless you get both halves, you're really sitting on a bunch of "could-have-been's." Obviously this is designed to sell more premium content, but those halves were only available at that time. It doesn't matter how much money you spend, you'll never get that other half. This is where a game like Fantasica is different. Trading allows a happier player base by creating loyalty and goodwill through the ability to exchange unwanted goods.
Another element of Fantasica that keeps it fresh, as compared to the Mobage/DeNa Transfromers game is content. In the Transformers game, "quests" are limited in scope and very uninteresting. They're really just screen-tappers with no value or interest once the quest has been completed. If you complete all quests, you're out of luck - there's nothing more to the game besides repeating the quests, or repeating the quests as part of the current event. After 10 months, there's still no new quest elements. The only new elements added were variations on the PvBoss and PvP elements of the game - and that's just not exciting. Fantasica has interesting events that tie into the quests - along with adding new quests regularly.
What I've noticed with the DeNa games is that they don't have much of a game behind them, tend to be nothing more that monetizing shells designed to capitalize on popular properties (MLP, Transformers, GIJoe, D&D,) and lack any depth or playability once you reach a certain point in the game.
Fantasica won me over (installed it as a MobaCoin promo) through it's depth and variety. It's also quite a lot of fun in the grand scheme.