My house is nearing 20 years, what will break soon?
It makes complete sense to replace your water heater early, because the failure mode for most of the older ones is "break and drain all over the inside of your house" -- and the 'drip pan' can only catch so much. 

You don't say where you're from, so I can't give you any climate or region-specific suggestions on what to replace and what not to replace. I know that down here in Texas, a "30 year roof" does not mean that your roof will last 30 years because there are too many variables... including tree cover, rainfall amount, temperature, what kind of tar paper they used on the original install... the best you can expect is to have to replace the roof at 30 years; the nominal life of a "30 year" roof when installed perfectly is about 25 years before you start risking the sheathing. Once it's degraded to the point where shingles crack easily, all it takes is one good thunderstorm to blow a bunch of shingles off. 

Depending on what kind of siding you have, the max life is also about 25 to 30 years -- especially if you have wood. I've replaced most of the T1-11 siding on my house with HardiPanel. It was worth it for the insurance discount as well. 

Insulation in your attic is PROBABLY insufficient compared to normal modern standards, and your roof probably does not have baffles installed at the soffits. This is something I'd pay someone to do; have someone come in and add soffit baffles (and increase ventilation if you can safely) and blow in insulation until you're at R-60 in your attic. I feel the same about windows and doors, actually -- they should be replaced after 20-30 years of service -- but we're veering back into energy efficiency here. 

Appliances are an obvious one. You can get by with not replacing a stove or a gas dryer, but washer, refrigerator, and dishwasher should probably all have been replaced by now with more efficient ones. 

The only other thing I can think of off of the top of my head that tends to go 'boom' suddenly is toilet and sink shutoff valves, unless they're the quarter turn or ball valve type. If they aren't, consider replacing them with ones that are quarter turn or preferably ball valve. 

Things I would have inspected or done yearly include pest inspection and treatment, chimney sweep/inspection, tree trimming if you have trees near the house, make sure your gutters are clear and clean and drain properly, and make sure that your yard is still graded so that water flows away from your house as opposed to towards it. Other things that haven't been mentioned but can blow up suddenly but can't be inspected for include foundation problems in slab-on-grade construction, garage door adjustment/spring, and issues with flooring if the flooring is still original to the house. 

What you should be pursuing is energy efficiency. In the process of pursuing energy efficiency, you will probably uncover many of the problems you would otherwise experience before they become critical. A good example is when I put new windows in; I discovered a mold problem and an old insect problem that had never been remediated ... and ended up tearing down half of my house to fix it!