the metal will dissolve under the influence of weak organic acids. the plastic will be eaten by fungi. but it will take a lot longer than 100 or 1000 years. a lot depends on the water status (how wet it is down there). some landfills are covered with a liner and some are in dry parts of the world. excavated 100+ years old landfills show that even paper products survive if it is dry.

So more than 1000 years from now, will that dissolved metal and plastic become usable compost by plants? If old landfills left dry, have they thought about making them wet before sealing them off?

I think there's a misapprehension here about what 'compost' and organic materials are. For the most part, it's stuff that was formerly living that is decomposed/decomposing, not 'dissolved.' And to simplify, that's mostly carbon, hydrogen, oxygen, nitrogen and small amounts of some other elements like calcium, sodium, phosphorus, potassium, zinc, iron, etc. (Someone with better chemistry background would describe this better, and I'm probably leaving some stuff out).

"Metals" etc are usable by plants only in very small amounts. While there may be some localised shortages/surpluses, they're not the core of the organic matter life cycles.

Anyway more to your question:

-Landfills generally are designed to limit water in and going through the system because you don't want bad stuff entering the water table. And while some metals etc are needed in very small amounts, many are not needed or poisonous in larger amounts.

-Plastics mostly consist of hydrogen and carbon. To some degree, they can be broken down by biological processes, but often not or only in specific conditions. But when buried, this would likely be very slow, and honestly, there is no shortage of either hydrogen and carbon - only usable forms of them.

-Since landfills are often sealed off and buried to various depths, it perhaps makes more sense to think of them as more akin to geological deposits. There are still some biological processes going on, but very slowly. (To the extent there is e.g. decomposition, some of the products like methane escape on their own). Remember, as it is now there is at least some effort to divert/reuse valuable stuff like metals before they get landfilled. And some of the contents of landfills are things like rubble and brick and even concrete that really are geologic in substance.

Pure speculation: I think it's far more likely that at some point in future landfills will be 'mined' for usable compounds (like metals). But that will be more about value and having chemical/industrial processes to extract them in usable form than about needing the organic materials. (There will be a need to deal with the other stuff in there though, including plastics and organics like paper)

Excellent post above. I have seen excavated landfills. Many of the earlier ones (1900-1970) were only a few feet deep, compared to today's very large specially designed ones that can be 100' or more of trash. A shallow one with no cap or liner had plenty of moisture going through. These often look like dark soil-like material with chunks of anything that doesn't decompose. Glass is another material that hasn't been mentioned. We (achaeologists that is) find discarded pottery and metals thousands of years old. Likely someone will be digging ours up some day.

They are more like geological deposits, that's a good term for it. And due to contaminants - metals, PCBs, a host of other chemicals slow to decompose - the resulting 'soil' will not really be usable as topsoil.

A few landfills are now being designed to decompose faster, in order to produce landfill gas at a higher rate to be used as fuel. Rather than trying to keep moisture out, they collect the leachate and feed it back into the top of the trash deposit, which enhances the decomposition. It also has the advantage of shrinking the trash faster, so you can add more on top. Landfill operators call this 'airspace' which they only have so much of in their permits (in the US that is), so they want to get as much in there as possible.

If you put too much AIR into this process, the landfill can actually heat up enough to create a self-sustaining anaerobic decomposition process, with the most rancid stench known to man. A very expensive headache for operators, neighbors and regulators alike. See Bridgeton Landfill, MO for a glimpse into that special hell-on-earth.

That's why I recycle like crazy. Landfills are generally good at what they do but should be used sparingly.