One new feature of note in 0.4c is the addition of mini-bosses. For the purposes of this article, I'll be discussing why mini-bosses can be bad for Roguelikes and what I'm trying to do to counteract that.
One major flaw with mini-bosses in Roguelikes is that they are often a static enemy. You know "x" mini-boss is going to be on "y" floor and so you can prepare by doing "z". While this technically only applies if you are spoiled and/or a veteran of the game in question, it is inevitably going to remove some of the random out of the random game. In 0.4c, every player who has beaten Floor 1 has likely ran into "The Destroyer". Those players will quickly realize "The Destroyer" is always there and he always has the same stats and abilities. Players are now inherently prepared to face "The Destroyer" (or skip him) every time they start a new character.
But wait, is there a positive to this "lack of random" element that is present in mini-bosses? I think there is some benefit. They can provide players with a tiny bit of predictability in a world that is otherwise random. That's what I like to call an anchor. The way I see it is that an "anchor" is any part of a Roguelike that the player is in complete control of, without question. For example, the player picks their species and background, and so those are anchors. Two things that aren't anchors would be enemies and loot. The player has no real control over whether or not he faces 20 Cu Pas on floor 2, nor if he only acquires blasters for loot. The constant anchor the player has is decision making. A player can try to think of the best way to deal with all of those Cu Pas, or try to make his/her character blaster-focused.
My (future) solution to this problem? Mini-boss pools. Each floor will have the capability of spawning different mini-bosses each run. I think this is a fair compromise between the flaw and the benefit: players will learn that each floor contains a mini-boss, but they won't know which mini-boss it will be. They might face every mini-boss on every floor across so many runs, but there will still be a small element of random there: you will never know exactly which mini-boss is on the next floor, only that there will be a mini-boss.
Another possible issue is making mini-bosses overwhelmingly challenging compared to the rest of the game. I'm taking a two-step solution to this problem. First, mini-bosses are largely optional: They all move slower than the player and they do not wander around the map (unlike every other enemy in the game). Secondly, I'm trying to balance all of the mini-bosses, especially those in the first three floors, against a large variety of characters. I think the first few mini-bosses have to be balanced around the player having nothing but a stimpack and the starting weapon. That will make them really, really easy during certain runs (you've found a great weapon and an abundance of stimpacks) and pretty rough during certain other runs (you've used up all your stimpacks and are generally under-prepared). I've started working towards this goal in 0.4c (stimpacks cure poisons now, which is a major weapon used by at least one mini-boss), but it will probably be a long time before I get it just right. Feedback can help a lot in this regard.
You can expect to see a great deal of development work done towards mini-bosses in the next version (0.5a).