Mystery of Fortune 2 -- (note that this might actually be a pay game; I picked up a bunch of stuff discounted to free over the holidays, and I don't remember if this was one of them)
It feels like a cross between FFXII and an idle game, and is actually fairly entertaining to watch, but is ultimately, like most idle and tappers, aimless. There's no active gameplay; everything you actually do is navigating through menus.
You draft up to 12 units (no reason to do less, unless you can't afford it at the start), pick their classes, set their gambits, toss on whatever gear you have, and start idling through stages. Use your money and loot from each stage to level them (and unlock additional classes), and repeat! ---Wait, gambits? Yep. This is the part I find more interesting. For people unfamiliar with FFXII, you could "program" your allies to do certain things in certain cases. For example, you'd probably stick Dead Ally->Resurrection and HP<50%->Heal at the top of the list, followed by Nearest Hostile->Stun and Nearest Hostile->Nuke 'em. This game has more or less replicated that, and you have five slots, with a conditional and an action, each, to program your team. Combos and classes are obvious at first, but when you start getting more advanced, your options open up dramatically, allowing for things like Highest Enemy Attack->Stun, Nearest Enemy->Knockback, Enemy with AoE->Confuse, Highest HP->Nuke, Lowest HP->Normal Attack.
...Though, to be perfectly honest, I'm not sure how realistically useful that is. It seems like you can level more or less infinitely (though getting successively more expensive), and, as far as PvE goes, the most practical solution is to just have some low leveled guys as support/whatever, and one ungodly powerful unit that can just solo everything in sight (example, my current guy is Lv1605... and the next highest is Lv148). Sure, it ain't fast, but it makes it easy to remember what classes need leveling to unlock your top tier classes, and get those critical skills. (High damage AoE on a short CD that also has lifedrain? Yep, he's never dying.)
Only one IAP-- it lets you run at double speed (4x, technically, but why would you ever not be running at 2x?), increases drop rate, and, most importantly, gives you perma 2x money. Which basically means you spend half the time waiting, which kind of makes this entire thing even more pointless, but hey~!