The obvious concern is unexpected oneboxing when you don't want it.

This is (never?) a problem in chat when a bare url is posted but applies even there in a multi-line message.

Everything that oneboxes is really silently converted into one of several markdown syntaxes, each of which can, of course, be used in a post to the same effect; it is just the automatic conversion that is restricted. So we can get your Wikipedia post with:

```
@@@ wikipedia 0000000000000000000000000000000000000000000000000000000000000000 PU4 "APL (programming language)" "APL is a programming language developed in the 1960s by Kenneth E. Iverson. Its central datatype is the multidimensional array. It uses a large range of special graphic symbols to represent most functions and operators, leading to very concise code.…"
```

which produces this (which is obviously in need of a border or some other css to make it stand out from the background):

---

@@@ wikipedia 0000000000000000000000000000000000000000000000000000000000000000 PU4 "APL (programming language)" "APL is a programming language developed in the 1960s by Kenneth E. Iverson. Its central datatype is the multidimensional array. It uses a large range of special graphic symbols to represent most functions and operators, leading to very concise code.…"

---

It isn't much fun entering all that manually. However, rather than automatically oneboxing links, I think it would be better to add another button on the toolbar which simply accepts a link in a popup and produces the markdown directly into the post.

---
*update November 2020:*

A big revamp of oneboxing that includes this feature is now underway. Progress can be tracked on this meta post:

@@@ answer 1677