The core rules have six classes available, and I wanted to find a balance between presenting the classic tropes, and reflecting this world. Classes are where you kind of put your stamp on the game, since it's the most visible flavor piece from a player's perspective.
So, there are three classic tropes: the fighter, the magic user, and the thief. These hew as close as possible to the source material, while still being the best fit for this game. They, and their abilities, are directly inspired by B/X.
But, the three others are unique to my world, drawing from multiple sources.
The warden is inspired by the druid, but it's not really a druid. It's more of a cleric of the wilderness, if that makes sense.
The bard is more aligned with role playing. Bards get chants, which work different from the other three kinds of magic. I have wanted to put chants in the core rules for a long time, and I found a simple way to layer them in. They are nifty and different. You don't lose anything by not using bards (and ignoring the class altogether if you really want to), but they add a different layer to the game that I like.
The friar is the last one I solved. Originally, it was just a re-skinned cleric, but I have shifted it subtly to be a bit more monkish. The friar has light armor and weapons, but gets to carry a quarterstaff. The friar attacks twice each round with the quarterstaff, damage increases every level, and it is always considered an enchanted weapon - so a friar at level 1 can hit anything with it.
The friar and warden reflect the religious realities of this world. The true goddess has died, and other things have stepped up to fill the void. There has been a return to worshiping nature, and the warden has emerged as an important religious leader that is trying to fill that gap. Friars hold on to the past, looking to restore the goddess. It's sort of a reversal of a classic paradigm. The contemporary worship is in the past, while more pagan beliefs are coming to the fore. The idea of a temple in town where a bunch of priests are sitting around isn't really a thing. The temples are old, many relegated to the role of museum of the distant past, with a few lonely caretakers.