As it stands, the game seems to go very quickly from missions that are "easy" and require little time to complete to missions that are "hard" and require either much more time to complete, or a substantial amount of time dedicated to "farming" idols beforehand to complete.
This can be pretty off-putting for newer players such as myself, as at first it feels very appropriately-paced, and then suddenly becomes very "grind-y" (only way to describe it). The effect it has is, we feel like we're progressing at a steady rate, and then suddenly we feel like we're not progressing at all in comparison. Discouraging, at the least, and makes me feel like this would be an easy game to put down.
(What I mean by that is, games like this, I think, operate heavily off of creating a certain "draw" - something that will appeal to certain kinds of people (like me!), draw them in, and, hopefully (for you), not let them go. As an idle game, the biggest draw, or the biggest "thing" that you're capitalizing on, is our desire to feel a constant sense of progression - that's literally what an idle game is: even when you aren't playing the game, you know you're getting further in it, yada yada, so forth and so on. So effectively eliminating that "draw" by making the game drop so suddenly from a "progression" standpoint is contrary to the nature of the very game itself. That's how you lose players! And 's why I feel that a post here is necessary.)
To get specific, let's look at the missions I'm referring to. The "World's Wake" campaign through "Your Mother's a Hamster" feels great. The "Descent into Darkness" campaign through "Dug too Deep" is perfectly fine. And on "Ghostbeard's Greed," "A Pirate's Life for Me" is alright. These missions, in that approximate order, represent a sort of "early beginner" to "late beginner" level - and it should be noted that it takes 14 missions to span this gap. They should be followed by a series of (probably 10-15) "early intermediate" to "late intermediate" leveled-missions.
Instead, of the ten remaining missions, we have one that I'd call mid-intermediate ("Double Trouble"), one that I'd call late-intermediate ("PAARRRfection"), two early- to mid-hard ("Lone Descent" and "Dual Descent"), two that span maybe late-hard to early-expert (or "very hard") ("Yo Ho Ho and a Bottle of Rum" and "Around the World"), two that are probably mid- to late-expert (haven't gotten there yet, so this is guesswork) ("Ms. Fizzle's Field Trip" and "X Marks the Spot"), and two that demand a new category - maybe "master" ("2,000 Levels for All" and "Shiver Me Timbers").
Anyways, you can see how quickly the game accelerates after the first 12-14 missions. The only solution is to add more missions, obviously, which I'm sure is something you're all working on - but the important thing is that you make these missions of appropriate difficulty to maintain the pace of the game for newcomers, otherwise you risk losing the players you draw in. I don't think you should worry about new additions for "veteran" players with dozens of thousands of idols UNTIL you've fixed the pace of the earlier game.
The only other option I see, if you don't want to make new intermediate-/hard-level missions, is to get rid of like 10 of the beginner-level missions, to set the pace for the game as "slow" - but at least a "consistent slow."
Slow pace -> slow pace -> slow pace is better than fast/medium pace -> slow pace -> slower pace, because, again, that's how you lose players. And, of course, Appropriate pace -> appropriate pace -> appropriate pace is the best-case scenario, but again, maybe you've decided there are better things to dedicate your time to (because it WOULD require lots and lots of new missions, and I have no idea how complicated the coding could be). Guess it's your call.