Advice on niche idea

Do a google search for your "sub-niche", are there ads at the top and left of the results page? If so people are spending money to advertise - that equals money to be made.

The best advice that I can give is to do your keyword research thoroughly first, it all starts with a keyword that you are likely to be able to rank on, remember that if you are in a hyper competitive market it may be difficult to get page one rankings, whilst you will get traffic organically from page two, page one from position 1-6 will get you a lot of traffic. I would recommend looking at your main keyword (use google keyword tool) check the box "phrase match" if global monthly search is under 1000 then its most likely not enough traffic in the search phrase. like ways search on Google phrase match "keyword" if this is over 60,000 competing pages unless you really know SEO it will be difficult. Remember to ensure your onsite optimization is perfect Bold the first instance of your keyword in your article, bold and underline another instance, itilicise another instance and underline another instance. Link to Wikipedia for the wiki explanation of your keyphrase to get an outgoing authority link. Make sure you do not keyword stuff the page or pages I would suggest around 3% but certainly not more than 4%. Finally don't forget to sprinkle a few LSI keywords throughout each post. just doing this will get the site to rank without backlinks, then using minimal effort strengthen the site with some backlinks, don't over do it in the early days and link from high page rank sites. Google plus one is getting more important although I understand that standard backlinks still work, I have seen no changes yet to my sites without +1 so there are mixed opinions on this one, but it is going to add extra link juice anyway. here's an idea if the site is looking good for conversions use Yahoo PPC to drive traffic to it, if you have google ads on the site you can drive traffic from yahoo, this was frowned upon previously but google don't seem to be slapping for this anymore, oh one more thing make sure you get video on the site it makes a big difference for both ranking and lowering bounce rate, which also is important to the SE's
I hope this helps, if you need some more help on this contact me on the forum, and don't worry about the niche, I will help and i promise not to steal it, that's why we are here - to help not to nick your idea's.

All the best - John