This isn't really the right subreddit for this sort of question. You might want to check out /r/althistory or /r/whatif instead.

However, Sekigahara was an extremely complex battle, not least because of Tokugawa's political shenanigans. More importantly, it took place far away from Tokugawa's powerbase in the Kanto region. A defeat for his forces would likely in him simply withdrawing to said powerbase and biding his time, as he commanded some pretty impressive fortresses in the region, including Odawara castle, which historically had never been taken by storm.

On the other hand, the loose "alliance" of anti-Tokugawa daimyo would likely have collapsed on itself. The Mori of Western Honshu (Chuugoku) and the Shimazu of Kyushu, for example, were strong rivals which only intensified after Tokugawa's triumph. As a result, I would expect to see a continuation of the Sengoku period for some time, without a clear leader to assume Hideyoshi's position.