What major problems should I expect if I keep the car another 4 - 5 years?

The same problems can occur on any car. It will just be newer. My '88 Grand Caravan has not needed head gaskets in 24 years and 380,000 miles. Some vehicles need them in less than 20,000 miles so it's obvious you can't make predictions.

Head gaskets at 10,000 miles are not common on any brand or model so the chance of needing them again in the next ten years is remote. The differential cover is not a big deal. Not common, but not expensive, and surely not worth considering trading in itself. You're going to have little things like that with any car. If you like the car otherwise, keep it. All cars need repairs and this one is not uncommon. The best thing you can do in the future is to replace the coolant every two years. It's the water pump lubricant and corrosion preventer additives that wear out over time. Also, acids form in it that promote leakage from head gaskets.