I have a UA revised ranger with a Wisdom score of 10. Technically he's a UA Revised Ranger, but I don't think it matters for this question.
I used point buy to create the character, and pumped all physical stats. I have not been attempting to optimize this character before now. I have an image in my head of what he is like – that is how I have picked his stats.
I was doing some level advancement planning and read ahead to his Level 20 ability, Foe Slayer:
At 20th level, you become an unparalleled hunter. Once on each of your turns, you can add your Wisdom modifier to the attack roll or the damage roll of an attack you make. You can choose to use this feature before or after the roll, but before any effects of the roll are applied.
This will give my ranger absolutely nothing. He will also not get any new spell slots or spells at level 20. Am I right in saying that the only thing he will get is a new hit die? He can't even multiclass, due to needing both Dexterity 13 and Wisdom 13 to multiclass either in or out of Ranger:
To qualify for a new class, you must meet the ability score prerequisites for both your current class and your new one, as shown in the Multiclassing Prerequisites table.
Am I reading this right? What's the best level-up option for a Ranger 19 with 10 WIS?
Just a reminder: my question is not about getting to level 19. It's about getting from level 19 to level 20.
Best Answer
Take the hit die
Your general assessment is correct, you'll only get the hit die when reaching level 20. Multiclassing is not an option for exactly the reasons you give in your question.
However, there's always epic boons
If your DM is nice he might give you an epic boon that gives you a permanent Wisdom increase. Both your spellcasting and Foe Slayer would become better by this.
An alternative is replace Foe Slayer
This is homebrew territory, but not unexplored homebrew territory. When you reached level 3 you had to pick a subclass and immediately got a subclass feature. These features tend to scale pretty well with level. Maybe you can get the level 3 (or other level, but 3 generally makes the most sense) feature of a different subclass than your own instead of your Foe Slayer.
In the end the solution boils down to the oldest trick in the book: Talk to your DM and reach a solution that is fun for all.