I can think of three possible endings
1. Natives get uppity and a coalition of all the most powerful native enemies Ainz has accrued come together and pose a serious threat. There are big figures like the Platinum Dragon Lord, the Theocracy and who knows what else, plus you had characters like Jircniv or Princess Renner who had viable plans to beat Ainz so it's not impossible for other sasuga NPCs to have viable plans against the guild.
Probably the least likely though; one thing Overlord's consistently shown is that the series is not about fairness or will x beat y. It's about family, and being the last MMORPG skeleton clinging to a dead game
2. Ainz Ool Gown infighting destroys the guild.
Some of the NPCs like Jircniv or the theocracy note that the easiest way to kill the skeleton king is if some of his generals turn against him, so this is a possibility which has been foreshadowed a lot. Whilst most of the generals have shown 110% loyalty to Ainz, there have been notable instances where it's not entirely clear
-Ainz telling Pandora's Actor to grow beyond his father is a classic staple of dramas where the son usurps the father
-The generals do not like each other and clearly differ on how much they value humanity. Case in point being Sebas vs Demiurge & Albedo, Ainz seriously entertained the possibility that Sebas would try to kill him
-It's left ambiguous whether Albedo is actually loyal. She makes reference to how she is loyal to Momonga, but he renamed himself to Ainz and every time he acts merciful to humans she comments he's not acting like Momonga. An Albedo coup-attempt where she tries to monopolize Momonga to herself (personal victory) whilst steering the sorcerous Kingdom towards a hostile anti-human stance (as she feels is proper and true to her conception of Momonga) is possible.
-We have had an Ainz vs Shalltear fight already
-Ainz is not the first time players have been isekai'd to their world before. Most of the times players arrived they either died natural lifespans, got assassinated, or notably conquered the world with little opposition - before dying to infighting. There is precedent for an unstoppable isekai'd army to ultimately be its own downfall in this setting.
-Thematically, it would be a tragic but not incoherent end to Ainz's story.
3. The story ends with Ainz conquering the world, sasuga Ainz Samas all around, the author is released from their stressful series