The problem is that doing this while working full time is freaking hard. Doing an entire year of development after hours is just brutal. The longer the time to launch the more likely you are to not see it through to the end. So if you're taking on a project that's going to be in development for several months before you can even charge a dime -- finding something to get excited about is huge.
If it's just something you're going to build in a couple of weeks then that passion can totally be lacking there because marketing is going to be much larger part of the effort (it typically is but typically not to this extent).
In any case, I highly recommend sticking with something you can launch very soon. Probably no more than 3 months. I went 6 months and it was tough. It's an exhausting effort so take advantage of that motivational boost that launching gives you.