How to identify a technically complex project?

Great question! I recommend going through these other Taro discussions first:

• "How to identify projects that are more suitable for senior engineers?"
• "How do you choose an opportunity for technical depth?"

On top of the advice I shared there, the most straightforward (but also sort of fuzzy) way to identify an opportunity that merits a senior promotion is to see if there's some big problem lying around that all the junior and mid-level engineers on your team are shying away from. If you're able to take on that tough problem and solve it, that means you're operating at a higher level almost by definition. From my experience working at larger companies like Intercom, these kinds of issues are often tricky bugs or internal infra problems (e.g. build speed is slow).

I also recommend this discussion from a mid-level engineer at Apple on how to come up with new, impactful projects for a team.

Lastly, if you share more context, I'm happy to give more pointed advice. That can include stack, product space, how much tech debt your team has, stability of your system (how's oncall and bug load?), and what's on your backlog in general.

One hack I recommend for engineers here is quite simple: just ask people. Since you're a mid-level engineer, there's likely not an expectation that you define your own scope at this point.

So one way to tackle this is to talk to 2-3 tech leads (or other very senior engineers) on your team and adjacent teams, and talk to them about the roadmap for the next 6 months. Most likely, they are aware of the (many) problems that your team faces, and they have more work than they have capacity to handle.

If you show that you're aware of the problem space, willing to work hard, and you can take initiative, you can likely just pick up the projects they recommend. There's still a ton of work in further defining it, setting up milestones, and actually implementing it, but this strategy guarantees the project is of sufficient complexity and impact.