Campaign Concept: Explorers and Traders of the Great River

[Yora]

I am feeling very good about the stuctures and mechanics that I have put together at this point. It doesn't really require coming up with anything new that wasn't already in in the little D&D Expert Rules 40 years ago, and Worlds Without Number is an excellent resource for quickly putting together little dungeons or groups of town NPCs that may be used for just one or two sessions.
If anything, reducing the wilderness from a hex-map to a simple upside-down flowchart actually makes everything a lot easier by having all areas clearly compartmentalized. If players end up getting completely stuck with no idea how to proceed, they can always fall back to "let's get back into the boat and head further upstream to see what we run into". Since there's only the two option of continuing the stream they are on, or going back to the previous fork and taking the other stream, things become very predictable for the GM which reduces the pressure of having content ready
This also should work fantastically well for a West Marches campaign. Players can easily form a new party at one of the many trade posts, and even if nobody happens to have a ship, they can always just grab two cheap canoes and are ready to go. And they can all contribute to expanding the big master map at the Last Outpost.

I think the main thing from this point on would be ideas for ruins and encounters that incorporate the river element. Though it takes only a one minute walk away from the bank to place down pretty much any adventure you could think of. Another point to expand on is how the culture of the local population could be flavored to reference it being a river society.
For example, instead of trade caravan, there are river merchants who are going up and down the river visiting towns and villages and provide mobile supply stores for parties in areas with river traffic. Miners and loggers ferry their resources to markets at the coast on big barges, which they then tow back to port with huge long-legged river crabs.
Making rice the main crops fits perfectly, and I am thinking of putting together a simple list of main fish and water birds that are commonly eaten. Maybe have explorers wear raincoats made from crocodile skins. I think there's a whole world of elements that usually don't get looked into much in most Medieval Western Europe style fantasy settings.

Being advanced Bronze age I am thinking Classical Greek, Hittite and Egyptian levels of tech and classes.

Yes, that type of stuff.

I'm going with the Worlds Without Number system, which is a quite extensively modified version of Basic/Expert D&D. Basically it comes down to Fighter/Thief/Wizard (with priests in the form of dual-class wizard/healers).

There isn't really that much to it applying it to various types of societies. No two-handed swords and crossbows and being selective about the armor types is really all I've done in that regard. Conveniently the system doesn't bother with race abilities and modifiers for PCs, but the main culture for the cities and rice fields near the coast is based very heavily on the Dunmer from Morrowind. The other three peoples were represented in my last D&D campaign as wood elves, goliaths, and tritons. The latter obviously having a huge advantage in water, but deeper rivers are often so murky they'll still be effectively blind.