Hi Therax,
I recently learned that vanilla Factorio does have 1x1 and 1x2 loaders, and loader-redux gave those entities graphics. However, they don't work with Wagons for some reason, and to make them work through Lua required running expensive calls to the Lua api every tick.
Do you know why the vanilla loaders don't work with Wagons while regular inserters obviously do? I suppose this is probably a question better suited for the game developers but I thought maybe this is something you would be familiar with. Also it seems strange to me that inserters would be able to effectively find the wagons when they enter the station while loaders are unable to. It seems like it should be possible to check the train's cargo wagons when they arrive at a train station for nearby loaders and unloaders, so it's unclear to me why that would be such an expensive operation.

There's no fundamental technical reason for loaders to be so restricted. Wube stopped working on loaders when they decided to introduce stack inserters, and they haven't been worked on since, really. They were planning on removing them entirely but left them in for modders to play with, but they just haven't gotten any attention from Wube since they aren't part of the vanilla game.

That's interesting. Personally I find loaders attractive because I don't find the problems arising from stack inserters very interesting while they also require more entities which leads to less UPS and more convoluted designs than would exist in reality (especially for train loading and unloading, I like your bulk train loading mod btw). I'm considering making a feature request to add wagon support for loaders if it doesn't take too much time from the developers.
I have some mod ideas for enabling cleaner late game builds and for a lot of them loaders seem like a natural fit. For example 4x4 assemblers whose inputs and outputs are underground belts running underneath them that come out the other side. I haven't implemented a mod for Factorio before and I thought that this would make for a fun challenge however I'm unsure if something like that is feasible as a mod. Also most people would probably consider that overpowered and unfun but I'm interested in the sorts of designs it would enable.

Check out Whistle Stop Factories. Also Loader Furnaces, and of course the Chute from Miniloader is intended for this purpose in a roll-your-own way.

One of the Wube devs has said he wants to rework loaders one of these days, but I believe they’re busy with work on the expansion and will be for the foreseeable future.

I have some mod ideas for enabling cleaner late game builds and for a lot of them loaders seem like a natural fit. For example 4x4 assemblers whose inputs and outputs are underground belts running underneath them that come out the other side. I haven't implemented a mod for Factorio before and I thought that this would make for a fun challenge however I'm unsure if something like that is feasible as a mod. Also most people would probably consider that overpowered and unfun but I'm interested in the sorts of designs it would enable.

There's a mod that turns Factorio buildings into more of a Satisfactory style version, so basically everything has built in loaders and unloaders.

As for the vanilla loaders, the best idea might be to take the Loader Redux mod and create a kind of base-mod that upgrades the vanilla ones which then can be used by other mods to build up on?