Existing Market. 
Step 1: 
I've mentioned this in previous posts. The main focus of my business plan is to provide the service of one on one coaching for clients with specific goals, 100% dedication, and the financial means to afford coaching.  Since this is a small niche, I plan to differentiate my offered services by selling non-customized workout plans on my website.  This product will still apply to the same market- people with fitness goals- but it is a cheaper option that requires less dedication, commitment etc.
Step 2:
I spoke to three friends of mine who all have very specific fitness goals and currently have, or have had coaches in the past. When I explained to them my business plan (which several coaches/social media influencers have already proven to be successful when executed properly) here are what they suggested was next for me:
Merchandise -  One of my friends mentioned selling branded merchandise.  Again, imagine Steve Cook selling swoldier nation T-shirts and bicep blasters, or Katy Hearn selling resistance bands and hoodies.
Ebooks- Another friend suggested selling Ebooks such as nutrition guides, How to track your macros, what to know before competing in the NPC, and other relevant fitness subjects.
App- The last friend suggested creating an app with follow-along workouts, or an interactive discussion board for fitness questions/answers.
When I explained offering both one on one coaching and the less epensive, one time purchase workout plans, they said it was a great idea.  They all agreed not everyone needs or has the money to pay for a coach so this is a great option to offer.  They also said that I can do that, and sell merchandise, Ebooks, or have an app, in addition to my plan, (and one day I hope to offer all of the above.)
Step 3: 
I believe all of these ideas are relevant and make sense in my business venture, and existing market.  I think over time as I build social media platforms and gain a following/audience I can slowly incorporate these ideas and further differentiate my offered products/services.

New Market.
Step 1: 
Since my existing market is those who have very specific fitness goals, have the time and dedication to pursue those goals, and have the financial means of doing so, the radically different market would be overweight individuals, maybe people who have never been in a gym before, who don't understand the general concepts of nutrition, and don't have financial means to hire a coach.
Step 2: 
To target this market I could (this is pretty out there but hang with me) travel to big cities/highly populated areas and hold free fit-camps... For example, hold a free workout session, with a Q&A seminar, and possibly even partner with healthy food services in that area and/or supplement companies to give out free samples.  This would provide value to this market by providing education about health/fitness and available services in their area, as well as serve as a marketing tool for my personal brand and the businesses that participate.
Step 3: 
I spoke to two "un-fit" individuals who I think would fit in this market and one suggested offering  a free week of workouts, or few free weeks of coaching for them to see if they like it and can stick to it before they purchase something.  I thought this was a good idea that could easily be done.
The other individual said my plan sounded good, and that they would be interested in something like that in their area.  They did not make suggestions as to what to adapt about my business plan.

Reflection:
Before this I hadn't truly considered this market.  I always thought I could help the overweight population get healthier and lose weight, but I didn't think about strategically marketing them since my personal interest is in competing and that's a very small and specific niche.  
Surprisingly I do think this market is as attractive as my existing market, because theres the potential to really change and impact peoples lives in a big way.  I will definitely consider this moving forward with my business plan and try to offer lots of free and introductory options and try to market this market more (lol).