I'd love the ability to create bock beers by adding Hops to Lager in a brew kettle with increasing strengths of intoxication.

• Lager + Hop = Bock
• Maibock + Hop = Doppelbock
• Doppelbock + Ice = Eisbock

This mechanic would be roughly in line with how these beers are created irl.https://en.wikipedia.org/wiki/Bock

The crafting with Ice would be interesting, as it is not really a brewing recipe, involved heat.

I had once an idea of having beer and wine glases as placeable blocks. Putting ice into them could be an option.

Brewing is always interesting 😄However the trick is not just adding ice itself, to quote the article it's beer "...that is made by partially freezing a doppelbock and removing the water ice to concentrate the flavour and alcohol content..."

I thought using an ice block would be easier as a crafting recipe. A more involved brewing method of placing ice next to or underneath the brewing kettle would be preferred.

Underneath the brew kettle, replacing the fire block, makes the most consistent sense.
Secondary output would probably be a snowball.

Or what about placing the fluid in cold region to make it freeze. Breaking the ice and then retrieving the new fluid again?

I agree with Syndaryl on this one. The problem with making it cold region dependent is 2-fold:

1. With certain maps and large biomes you wouldn't necessarily have nearby cold biomes.
2. If you brew Maibock with Hops to get Doppelbock in a cold biome it would automatically freeze to get Eisbock.

There's probably ways around number 2, but the biome dependency seems like more effort for less benefit to me.