Map design

Well, if it's a long-term project, it's better to choose just one. After all, your own project is more important ;)

Yes, of course, but it's useful to take a break from your own map (it's actually beneficial for your project). It gets boring to work on just one map, but when you change direction and then return to your map (so to speak, with renewed vigor), the result is even better than you expected.

In theory, you can properly distribute the workload if you divide everything (design, gameplay, scripts, quests...). It might hurt your ego a little, but it will result in more good maps.

I partially agree, but when you work on something collectively, many things have to be redone later on. Each person contributes something of their own, and this "something" doesn't fit into your original idea. I once worked on developing an online RPG game in this way. We recruited a team of 10 people and started writing a Dungeon from scratch. As a result, the work went faster, but the final result took significantly longer compared to if you were working alone (because you have to redo things). So, there are both pros and cons