After recently showing off its second stealth fighter plane there are increasing gossips about it's new stealth fighter the J-31. It took 4 years more for the J-31 to be introduced to the public than the J-20 but it seems like that the former will be introduced before the later.
Now before we go on and assume who will buy them we need to keep in mind that the J-31 is still far from entering service. It has yet to be tested (at least publicly) with Chinese engines (WS-13A) and a realistic date for it would be 2020.
That gives 6 years and within that time military will improve will become more powerful especially that of developing countries.
So, here are the possible users:
1. PLAN: China has been intensively increasing its naval capabilities and modernizing the People's Liberation Army Navy (PLAN). There are plans to add more aircraft carriers ton the current one and PLAN is aiming for 4 by 2020 (will definitely get 2). The J-20 is twice the weight of the J-31 and might not be possible to launch it from an aircraft carrier but the lightweight of the J-31 gives it an edge. Most likely scenario would be the J-31 will be manufactured for PLAN whereas J-20 for the air force (PLAAF)
2. Pakistan: There are numerous reports that Pakistan will purchase the J-31 and possibly acquire stealth fighters before India. Given the good relations between Pakistan and China the deal makes a lot of sense. The number of jets Pakistan will acquire depends on the Government's purchasing ability and price of the jets.
3. Iran: The Iranians are desperate for new fighters to its ageing air force inventory. With increasing oil demands (demand for oil will hit 99.7 million barrels per day, up from its 2012 level of 89.2 million bpd) and countries like Saudi Arabia and Norway at it's oil peak and others running out of oil and Venezuela unwilling to deplete its reserves rapidly like Saudi Arabia the world will have to look at country with the World's 5th largest oil reserves to increase their productions. International sanctions will mean less then than it is now.
4. Brazil: China and Brazil don't have any conflict of interest, the Brazilian defence budget being a massive $30+ billion and it's willingness to acquire modern fighter jets and although it wanted the F-35 Lockheed Martin rejected it and presented a F-16 flying falcon (F-16BR) which isn't a stealth version. Due to stealth aspirations Brazil might switch its focus to the J-31.
5. Saudi Arabia: Israel's accusation of the F-35 by 2016 and yet to receive any words from the US about sale of F-35 to the Kingdom and even if a sale is made it is expected to be delayed by 5 years than that of Israel's. Also the fact that the Middle Eastern countries are made to sign deals that prevent them from using US weapons against Israel doesn't go well for KSA's interests. Saudi Arabia had previously purchased advanced missiles from China (DF-3A in 1987 which is a nuclear ballistic missile and DF-21 in 2007). The positives from deals with China is that there are no strings attached.
6. North Korea: South Korea recently signed a deal with USA for 40 F-35s and this might force the North to get its own stealth arsenal. Also China might arm North Korea just in case of a proxy war for the South China Sea.
7. Other Middle Eastern countries: UAE, Bahrain, Qatar are well aware of the fact that their troops on the ground lack proper training to fight off possible Iranian offence (or counter-offence) or militias thus would depend heavily on its Air Force to take out enemy tanks, missile, missile sites and would need stealth fighters to avoid radars (in this case only Iran, militias don't generally use radars). Again US are yet give any assurance of selling F-35s.
NOTE: Malaysia, Indonesia and some other countries were excluded because they have conflict of interest with China in the South China Sea. Also Venezuela, Bangladesh, Argentina and some other countries are excluded because they would first look to modernize their Air Forces before going for 5th generation stealth technologies.