What is essential for a landingpage to validate your idea?

In general, your landing pages need five (5) things:

a) RELEVANT OFFER (i.e. PRODUCT, SERVICE, or anything of value to the market)
Depending on how good your lead generation targeting is, if a person is already on your landing page, then they more likely already fit the profile of your target audience. The first thing you need to do is to present your offer to them in a clear and concise manner taking into consideration the typical visitor's short attention span.

b) STRONG CALL TO ACTION
Make it undeniably obvious to your visitor which action you want them to perform to culminate their visit to your landing page. Make it unmistakeably easy for them to perform that action. And possibly most importantly, give them no reason to hesitate.

c) COMPELLING PITCH
Now to get your visitors to respond favorably to your call to action, you need to give them strong motivation. Aside from the regular benefits that they get from the product or service, highlight extra benefits that they can only get by performing your requested action. Limiting availability (limited time, or limited number of slots) can also add a sense of urgency.

d) ADDRESS CONCERNS / DISPEL HESITATIONS
Depending on the nature of your product or service, your market may have some strong concerns that cause visitors to hesitate or ignore your call to action. This can be something common to your product category in general, or something related to your call to action (e.g. requiring a credit card number). No matter what it is, if it prevents people from responding favorably to your call to action, you need to address it.

e) ANALYTICS (Tracking Components)
You need a way to capture the activity and behaviour of visitors on your landing page. Analytics will give you the insight you need to understand what's happening, make decisions, and apply changes as necessary.

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f) METRICS (Not exactly on the page, but I deem it relevant to mention here)
Identify which metrics are relevant in your situation for your goals & objectives. Figure out how to measure these metrics and what raw data you need to capture in order to compute these metrics. Your analytics system will play an important part in capturing data for your target metrics.

There are a few things that need to go on your landing page:

1/ A clear message that tells people the exact problem you solve, and how solving the problem benefits them.

2/ An opt-in form to collect email addresses. Say something like "I'm currently in the process of building this to solve x problem. If you're interested in getting a massive 75% discount off the retail price, enter your email address and I'll let you know when it's live."

The idea is that if enough people give you their email address even after you've told them it's not going to be free, it's a sign that you might be on to something worthwhile to build.

To convert paying customers (who hang around).

Or if you don't have payment stuff made yet, to get them to sign up to something. Though them signing up to something (like an email) doesn't really validate the idea, it's more just a positive sign.

Would say this depend on your audience. If you want to sell to enterprises your MVL (minimum viable landingpage) differs from indie hackers.