You should start with a niche and stick with a niche. Gaining topical relevance through indexing and interlinking is a big help, and diluting that doesn't help. If you simply must expand further, stick to one vertical.
For instance, if you feel like you're missing out on opportunities to grow and you have the resources to do so, because you've completely tapped out the cardio-for-women nice with running, Zumba, BodyPump, etc., then you could expand horizontally to yoga and stretching and light weight training. But I wouldn't start bringing nutrition into the mix or parenting and pregnancy.
Stay in the umbrella of Fitness and during your expansion keep it tight nit. I wouldn't bring in body building and strength training and especially not fashion and makeup, of course.