An Anki2 addon by eshapard which I made to work with Anki21. The addon automatically adjusts the new interval to target an 85% review success rate. A more in-depth description can be found on eshapard's code blog.
While eshapard's code blog mentions that the addon might work with Anki21 if you comment out a particular line, I discovered that lapses are rarely registered as such, and so the addon only adjusted the new interval on the first time you run it, and never again, because even if I lapsed 50 cards, they would not be detected on Anki21.
I don't know if this is due to the Anki21 migration, an original bug in the Anki2 addon, or the author's intention. From reading his post about how he intended it to work, it is very likely it is one of the first two. What I did was modify the SQLite queries to correctly detect lapsed cards and correctly calculate the success rate. I also added the option to allow the addon to work in the background without informing the user of the changes it made.
Create a folder called autoLapseNewInterval inside Anki2/addons21 (usually located in C:\Users\Username\AppData\Roaming on Windows) and place __init__.py inside that folder. If you're using older version of Anki, make sure to use one of the legacy releases appropriate for your version instead.
For more in-depth information on what the addon does, you should read eshapard's original post (backup).
You can adjust the change_silently variable in the code settings to allow the addon to make changes silently without prompting you.