There isn't a simple recipe for idle games, but they have to follow some basic rules in order to be fun. You need to be able to go almost infinitely and it needs to scale. The things you do manually in them are often more powerful than simply idling and it needs to keep your interest so you don't stop letting it run because you're bored of it. Tap Heroes doesn't do most of these things, and it's a weird idle game that couldn't keep my interest.
The core concept is you having a warrior, thief and mage to kill endless numbers of enemies. You can tap on your warrior to heal it and tap on enemies to damage them. Right there, the balance is weird, because while tapping to heal makes sense, the numbers for damage are completely unbalanced, enemies have tons of HP when you progress a bit, and healing is the only thing that makes sense. The rogue will slow enemies by some factor - unsure what the slow does, actually - and deal poison damage over time. The mage will heal your party and I'm confused as to why a 'mage' is doing the healing instead of a 'priest'.
You get gold for killing enemies, which can be used to buy upgrades, you also get gems from completing each 10th stage, from chests or from achievements. The achievements are crazy, by the way, I've spent a week full time on the game before getting bored and I had maybe 0.1% of the 'kill 10000000 monsters' one, never gonna get that. These gems can be used to buy more numerical upgrades for your heroes, or special things like a familiar that attacks your enemies, a banker - that doubles, then triples your cash - ad a inn that fully heals you between stages. All of these things are weird in their own way, I had a bug where I couldn't buy more levels of the familiar - there are three - the banker feels dumb because it stops at x4, why? It's an infinitely going idle game, it should go to infinity as well!
The skills you can buy with gems aren't that fun either. They have no upgrades, work for X seconds and then go on a lenghty cooldown. I wish there were ways to make them better. But that's an issue in general with TH, it sets itself up for obvious limitations. The stats you can upgrade like speed and critical hit chance were set from the get-go to have a maximum limit. I felt that even upgrading everything evenly, I couldn't match bosses and I had to grind and click to heal for a few minutes to kill them, so how did I feel after maxing certain stats? I felt that I would need much more grind to get there. Maybe stats could've increased without ever getting to their maximum, maybe when you maxed them you could've increased something else - critical damage modifier, chance of double hit, etc.
TH doesn't feel like it was built to scale, even the gold counter can't go higher than 999999 (you'll still have more gold, but you won't be able to see it). Why not use scientific notation? Anyways, that's what I have to say about this game. It was okay for a while, but then there wasn't anything new to unlock, so my attention dried out.