I expect the EU will start admitting new countries to make up for the expected lost revenue in 2018. 
The only countries likely to be admitted anytime soon are the ones from the former Yugoslavia. These are all tiny, and some of them are basket cases. No way will they contribute to any Post Brexit EU finances - infact probably quite the opposite.
The post Brexit UK however will be contributing a quite healthy amount via WTO tarrifs and import duties 
That won't go anywhere near covering their export losses to the UK as any tariffs will be reciprocated and they sell more to us with their anti-UK bias leading to a higher and higher negative trade balance. The EU will have to compete against the many bilateral trade deals with zero or very low tariffs with 12 countries that have already contacted us. This includes USA, Canada, Ghana, India, Australia, New Zealand and South Korea. Apart from South Korea the are all major cheap food exporters. Going long on shares in EU food storage warehouses looks like a very profitable strategy.

These are some of the areas that the EU have stuffed our trade balance with them. CFP so as an Island we used to be net fish exporters. Now majority of fishing is by EU boats through EU quotas, so we are net importers. EU milk quotas means that where we used to be self sufficient we are now major importers. Energy we import 24/7 about 10-11% of our electricity where we used to only do this at rush hour peak time due to EU energy policy. We have lost most of our energy intensive heavy industry due to EU green tree-hugger energy rules including aluminium, steel, glass, cement and oil refining. Germany have managed to get away with cheap electricity to industry (to stop them relocating to Poland) by loading the extra costs on domestic tariffs. EU have launched a competition inquiry that is due to report in some century soon. France is the industry subsidy king. normally on investment in Green development stuff and they get away with it. A single market in services, on of the UKs strengths has been largely blocked by France and Germany. Fortunately, this will now change, I'm personally looking forward to cheap New Zealand lamb and butter again.

I expect our EU trade balance to now improve (we will save £10.8bn and rising in net fees). We will export less (but this is falling anyway), but imports will fall much much more with higher domestic production and cheaper low tariff substitutes from global partners.
