Learn Arabic (Modern Standard): لنناقش الخطة أولا.

It uses the prefix لـ (li-), known as the "lam of command". When you attach this prefix to a present tense "we" verb like نناقش (we discuss), it changes the meaning from a simple statement to a suggestion: "let's discuss".

Could I use دعنا (da'nā) or هيا (hayyā) to say "let's" here instead?

You could say هيا نناقش (hayyā nunāqish) meaning "come on, let's discuss," but it sounds more like an urgent rallying cry. دعنا نناقش (da'nā nunāqish) is grammatically fine but can feel a bit literal ("allow us to"). The لـ (li-) prefix used here is the most standard and natural way to make a suggestion in MSA.

What verb form is نناقش (nunāqish) and what does that tell us about its meaning?

It is a Form III verb, which you can spot by the long "a" sound in its past tense ناقش (nāqasha - he discussed). Form III verbs often describe an action done with or directed toward another person, which fits perfectly with the interactive nature of a discussion.

Does the لـ (li-) prefix change how the verb is pronounced at the end?

Yes! This specific لـ (li-) puts the verb into the jussive mood. For a standard verb, this means the default final "u" vowel (damma) becomes a silent pause (sukun). Even though it is unwritten here, it is pronounced li-nunāqish, not li-nunāqishu.