Pretty simple, really. There would spawn tin and copper around in minecraft, and iron/diamond would be even more rare. To create bronze, you'd have to smelt 3 copper ores, and 1 tin ore together. This could be done in a new furnace with a 2x2 grid.

That grid could also be used in future updates when you need to smelt several ores/things together.

copper would also naturally be 3 times as common as tin.

If it makes sense, there could be copper tools as well, like in Terraria. But I'm not an expert on that field, so I'd like your input on that one.

The rules asks if there are downsides. I guess some people wouldn't like more stuff because they like minecraft being simple, I don't agree with that personally, but that's why it's just a suggestion.