If you think of traditional RPGs in which you gain levels and raise your basic stats, although you increase your power in absolute terms (higher numbers), it remains the same in relative terms, because the stats of the enemies increase throughout the game as well. So, although it works as a motivational factor, there's a certain illusion of progression in games like that.
It's only an illusion to a certain extent, because not only you get higher stat numbers, but different skills, more items, more party members, game options, etc., so there is an actual progression in the game. But the diversification of the challenge increases as well, and even though you get stronger and learn to play better during the game, the final parts are almost always harder than the initial parts of the game (i. e. enemies have complicated defenses, use tricky skills, etc.).
It's not really a flaw. I'm just writing this to introduce how I dealt with difficulty in Polymorphous Perversity.
First, I have to say I don't like hard games. I don't like games in which you die a lot and have to retry the same bits lots of times. I don't like it, period. And I don't make difficult games. If you played Beautiful Escape: Dungeoneer, you'll know it's an easy game. Very hard to actually get a game over. But though you can finish it with little effort, you'll need more thinking to get good scores.
I'm using a similar approach in Polyperv. With twists.
Disclaimer: I'm using the terms battle, enemies and damage because I am (and you probably are) more familiar with them. But like I said before, there are not actual battles, and they are not really enemies.
I described the battle/sex system in a previous blog. Finishing battles is fairly easy, as you can just keep using the basic fuck command and rest/use items to replenish stamina if necessary. Still, if your stamina reaches zero, you get no XP but you don't die either. The problem is: mashing the fuck command will probably get you a bad score.
Getting a good score involves some practice, and learning new moves. That means you're likely to get bad scores on all initial battles. Only later in the game you'll be prepared to get really good scores, even on the initial enemies.
For that reason, overall there's no clear increase in enemy difficulty through the game. Some enemies are harder (they have more stamina, use more skills, act more often, etc.), but the harder enemies are not necessarily at the final parts of the game.
My thinking is: finishing off battles quickly is so easy (and gets progressively easier) that players will start wanting to get good scores. Then they'll start thinking, and looking for tools to do it. And if they return to the enemies they fought early in the game, those same enemies will be harder... in a subjective way.
There's also a little problem: the main damage stat is potence, which determines how much damage your fuck attacks causes. As you increase your potence, battles will finish off more quickly... which is not always good. There are ways to reduce your own potence inside the battles, like masturbating, or repeatedly using the fuck command, etc. That's another element that makes the game harder/more strategical as it progresses.
There are bosses of course, and they are hard. But if you fail in them, it doesn't make a real difference story-wise (most of them).
There is a part where I went traditional though. Final parts of the game have a higher degree of challenge... not only with more bosses, but with less ways to rest between battles. If you fail a battle, your horny meter increases, and if it goes too high, you get a real game over. If you don't worry about good scores during the whole game, you'll get very little XP, your character will be weak, and you'll be unprepared to deal with the real final challenges.
That's not really an alternative way of increasing the difficulty of the game over time. This is my way of having the game be as difficult as the player want it to be. If you want it easy, easy it is. If you want complicated, there is fun for you too.