Lots of people truly believe now that the era of major, state-on-state warfare is over forever because one hasn't happened recently. I think that's absurd, and that we're just experiencing a lull in the fighting. Most reasons people give were also given before the War to End All Wars, and look how that turned out.
Possibilities:
1. Israel, Saudi Arabia, the UAE, and possibly or probably the US vs. Iran and Hezbollah. Quite a few people seem to be itching for this. Not sure how Turkey, Syria, Iraq, Qatar, and Bahrain would figure into this. Russia could also get involved.
2. India vs. Pakistan, over a combination of Kashmir, drought, and domestic political issues requiring looking "tough". As always for the past 25 years, this could go nuclear, and become the deadliest war up to that point. China could intervene on Pakistan's behalf in some way.
3. Russia, despite its its small GDP, is actually far stronger than NATO in Europe. Most European militaries are crumbling due to a refusal to pay for spare parts for their equipment, so the numbers of tanks, aircraft, and ships they have on paper is far greater than what actually functions anymore. If he fails to flip European governments to his side by helping install friendly and pliable parties, and if he feels cornered, Putin could quite easily seize the Baltic States within a few days, rush troops to heavily fortify it, and dare NATO to do something, safe in the knowledge that it would take months to assemble any credible force to retake them. It's also very doubtful many countries in the West would have publics willing to stomach tens of thousands of deaths, especially if Putin made it clear he considered the Baltic States territory of Russia to be defended with "nuclear de-escalation." People scoff at this now, but then, in 2013, people actually believed that the age of countries invading each other and annexing each others' territory was a thing of the past, and look how that turned out. Doing this would prove the inability and unwillingness of NATO to defend its members, making membership unattractive and potentially causing the collapse of the Organization. It would also give Putin a major popularity boost, help him reclaim some of the Soviet Union's level of power, territory, and prestige, and create a direct land bridge to Kaliningrad. And it would be quick and easy.
4. US-China relations continue to deteriorate, China strengthens, and Taiwan grows distant. This could lead to a PRC invasion of Taiwan someday. If the US intervened, it could get brutal, and US carrier groups are not invulnerable and cannot just bomb China like it's Iraq.