It might seem hard to find out what to build next but it's actually not that hard once you prioritize process over the finished product.

I'm assuming you want to start this as a side-business initially and not your primary business (this is important because it drastically alters the way you choose: e.g if you need money, you might choose something that isn't fun to build).

Obviously what you build should be something:

your comfortable building with (technically, emotionally and morally)
where you can learn something along the way
that isn't too easy but should be just hard enough to be challenging (this is called "progressive overload" and is the best way to get better at something)

The second thing to decide is what type of business this should be. I mainly differentiate between passive-income hustles and high growth startups. Passive income hustle idea sizes are around 1-3 while high growth startup idea sizes start at 7. Let‘s take a closer look.

There's a scheme (credits: YC) to decide which idea is worth pursuing:

How big of an idea it seems to be: 1-10 (10 would be above billion dollar valuation, think uber, stripe, etc.)
Founder / market fit: 1-10
- Should be in your circle of convergence
- How familiar you are with the topic
- E.g if you’re working as a waiter, you know exactly which Problems restaurants are facing. If you try to build an app for restaurants and aren’t really familiar with these yourself you should probably choose something else
How easy is to get started: 1-10 (attention: just because it is easy to get started doesn't mean it's easy to make money)
Early market feedback from customers: 1-10
Overall score: 1-10 (sum up all numbers and divide by 4)

Examples

Menuver (digital restaurant menu)

How big of an idea it seems to be: 3 (not too big, probably several hundred to thousand restaurants could use it)
Founder / market fit: 7 (I got a good grasp of how restaurants work)
How easy is it to get started: 8 (incredibly easy, just build a vue app with firebase and you're done)
Early market feedback from customers: 4 (pitched the idea several times, some people liked it, for others it was just meh)
Overall score: 5.5

Online Shop Building tool

How big of an idea it seems to be: 10 (seems really big, think shopify)
Founder / market fit: 7 (I'm fairly familiar with online shops, my dad runs an ecommerce store and i do all the tech)
How easy is it to get started: 8 (testing hypotheses and even creating an mvp shouldn't take too long, people would probably not be able to sell things right away because of all the legal stuff to consider but in general it shouldn't be too hard to start)
Early market feedback from customers: NaN (haven't tried it out but would guess 5)
Overall score: 7.5

Course Buddy

How big of an idea it seems to be: 8 (education is a hugely relevant topic right now and we'll see a lot of companies emerge in the future)
Founder / market fit: 10 (I love online courses and been to 3 universities, I got the perfect age to both understand the problem and think about the bigger vision)
How easy is it to get started: 10
Early market feedback from customers: 5 (launched an mvp, got good reception)
Overall score: 8.25

Push Notifications for Social Clubs (club managers can easily check attendance)

How big of an idea it seems to be: 2 (not to big but might be a very easy side hustle
Founder / market fit: 10 (I'm in a jiu jitsu club and know how hard it is to check attendance)
How easy is it to get started: 10 (incredibly easy, checking hypotheses with the manager is really easy and have contacts to other managers as well)
Early market feedback from customers: 8
Overall score: 7.5

This schema is very flexible and you can definitely add metrics to it, e.g "How fast does this make money?"

How to measure the quality of an idea

Examples