Learn Hardcore Chinese: dāngrán kěyǐ, wǒ zài wàimiàn děng nǐ ba.

当然 adds friendly emphasis (“of course; naturally”), and 可以 answers the underlying yes/no about permission or acceptability (“it’s OK; you may”). Together 当然可以 sounds like “Of course, that’s fine!” It’s warmer than just 可以.

Does 可以 here mean permission or ability? How is it different from 能 and 会?

Here 可以 (kěyǐ) expresses permission/acceptability (“may; it’s okay”). Rough guide:

可以: permission or possibility under rules/social norms. 我可以走了吗？(May I leave?)
能: ability/circumstantial possibility. 今天下雨，我不能去。(I can’t go because of circumstances.)
会: learned skill or likelihood. 我会游泳。(I can swim.) 明天会下雨。(It will probably rain.)

What does the sentence-final 吧 do?

吧 (ba) softens the statement into a mild suggestion/decision that invites agreement: “I’ll wait for you outside, okay/then.” Without 吧, it’s a more definite statement.

Can I replace 吧 with 吗?

No. 吗 makes a yes–no question (e.g., 我在外面等你吗？ = “Am I waiting for you outside?”), which doesn’t fit the intent. If you want to ask for confirmation, use something like 好吗？/ 可以吗？ after your proposal.

What changes if I drop 吧?

我在外面等你。 becomes a straightforward, firm statement (“I will wait for you outside”). With 吧, it’s gentler, like a proposal: “I’ll wait outside, okay?”

Is 在 here the progressive “be doing,” or just “at/in”?

Here 在 introduces location: 在外面 = “outside.” It’s not the progressive marker. Compare:

Location: 我在外面等你。(I’ll wait for you outside.)
Progressive: 我在等你。(I’m waiting for you [now].)
Progressive + location: 我在外面等你呢/等着你。(I’m waiting for you outside [right now].)

Why is the place phrase before the verb? What’s the word order?

Chinese typically puts adverbials (time/place/manner) before the verb: Subject + (Time) + (Place) + Verb + Object. So: 我 + 在外面 + 等 + 你.

What’s the difference between 在外面等你 and 到/去外面等你?

在外面等你 focuses on the location of the action (“wait outside”).
到/去外面等你 highlights moving to that location first (“go to the outside to wait”). Use 到/去 when you want to emphasize the going.

Can I omit 在 and say 我外面等你?

Often yes in colloquial speech, especially with common place words (门口, 楼下, 这儿/那儿): e.g., 我楼下等你. Using 在 is the default and sounds neutral/standard; omitting it can sound a bit more casual/regional.

Is 等 transitive? Why no “for” like in English “wait for you”?

Yes, 等 is transitive: 等你 = “wait (for) you.” For “wait a moment,” say 等一下/等一会儿. If you want both the person and a short time, put the time after the object: 等你一下/等你一会儿 (not “等一下你”).

外面 vs 外边 vs 外头—any difference?

All mean “outside.” 外面 is the most standard/neutral. 外边/外头 are common in northern speech; 外头 feels a bit more colloquial. You may also hear 外面儿 with -r ending in some regions.

Any pronunciation tips or tone changes in this sentence?

可以 (kěyǐ) → sandhi: pronounced [ké yǐ].
等你 (děng nǐ) → sandhi: [déng nǐ].
吧 is neutral tone. Full natural reading: dāngrán kéyǐ, wǒ zài wàimiàn déng nǐ ba.

Could I use 会 instead of 可以 (e.g., 当然会)?

Not as a direct reply of permission/acceptance. 当然会 means “(I) certainly will” or “(I) can (as a skill),” which shifts the nuance. You could say 当然会等你的 (“Of course I’ll wait for you”), but as a polite acceptance, 当然可以 is the idiomatic choice.

Can I add 着/呢/了 to talk about waiting?

等着 emphasizes a continuous state: 我在外面等着你。(I’m in the state of waiting for you.)
呢 marks an ongoing action now: 我在外面等你呢。(I’m waiting outside right now.)
了 often marks change/completion; e.g., 我到外面等你了 can imply “I’m (now) going outside to wait for you” (change just took place). In the original suggestion, none are required.

How would I make this more polite/formal?

Use 您 for “you”: 当然可以，我在外面等您吧。 You can also say 没问题 or 好的 for a courteous acceptance: 没问题，我在外面等您吧。

Are there natural alternatives to 当然可以?

Yes:

没问题 (no problem)
可以啊 / 行啊 (sure) — casual, friendly
好的 (all right) All can precede the second clause: e.g., 没问题，我在外面等你吧。