I saw the latest changelog entry. Fuel can now be disabled via setting, that is a great idea, because there are some mods with a multitude of fuels (looking at you, Py).

What I'd like to see instead (or in addition) is that if a machine has all its fuel slots filtered, then only those fuels are added (for inserting fuel and for outputting spoiled fuel).

P.S.: because of ghosts use https://lua-api.factorio.com/latest/classes/LuaEntity.html#get_inventory_filter

That's a cool idea! We do add filtered inventories to the list of recommended items for trains and some other entities.

However, understanding that a filtered fuel means "nothing else" is a novel idea. We currently only add more candidates, but limiting the list is something we can do in many places. Like, why should we show stuff other than ingredients/fuel for an inserter into a crafting machine with a recipe?

Or at the minimum sorting by "most likely" is also an idea.

Only if all fuel inventory slots are filtered it does mean "nothing else". It's not about removing things from the list afterwards, it's more about controlled addition.

I guess right now you have something like this:

if setting says to add fuel
add all fuel (and spoiled items)

instead you'll need

if entity has at least one unfiltered fuel slot
add all fuel (and spoiled items) from the allowed fuel categories
else
add all fuel (and spoiled items) from the filters

I'm not certain whether similar logic should apply if the fuel inventory slots are not filtered and instead have fuel in them. Because fuel rarely changes, and if it does it's likely a mixed belt of input and you don't filter inserters for that. I really like the sorting idea, for that scenario I'd definitely put all the additional fuels at the very bottom of the list.