If you're struggling to think of more keywords people might be searching about a specific topic, go to Google.com and take a look at the related search terms that appear when you plug in a keyword. When you type in your phrase and scroll to the bottom of Google's results, you'll notice some suggestions for searches related to your original input. These keywords can spark ideas for other keywords you may want to take into consideration.
The limit on manual keywords could be higher.  I personally wish we could input 10,000 keywords at a time, instead of 200.  However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input.  Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).
So whilst Long Tail Pro has just continued to grow in revenue every single year and comes highly recommended, you may want to check if all the important features are fully functioning before signing up. Especially the KC feature (keyword competitiveness) as it is so fundamental to keyword research and making an informed choice on whether or not to go into a particular niche.

Depending on your topic / vertical and your geographic location the search engines may have vastly different search volumes. The tool can only possibly offer approximations. Exact search volumes are hard to find due to vanity searches, click bots, rank checkers, and other forms of automated traffic. Exceptionally valuable search terms may show far greater volume than they actually have due to various competitive commercial forces inflating search volumes due to automated search traffic.
This tool allows you to type in your keyword phrase or keyword in the field that is provided and it will generate a long list of suggested keywords.  It has three button level and you can perform a deeper search by clicking on button level 2. You will get a list of keywords or keyword phrases that are related to the original list provided in level one. If you want to continue further, you can click on level three and then click the link at the bottom to download the data. The tool can assist you to discover those keywords that your competitors could be ignoring and use those opportunities to create fresh targeted content.
3. Ninja Outreach: Full disclosure this is my own tool, and it is actually an outreach tool, so you may be wondering how it plays into Keyword Research. The fact is there are quite a few data points that NinjaOutreach gets for me that I find useful in keyword research, such as the articles that are ranking for the keyword in Google, their domain authority, their page authority, the number of backlinks they have, and other social and contact data. It's pretty valuable stuff, especially if there is going to be an outreach campaign tied into the keyword research. I wrote a great article with Jake from LTP showing the combination of the two tools.
I was paid approximately 2/3 of the funds upfront and will receive about 1/3 over the next 2 years.  It's now been almost 7 months since we closed the deal, and I've received the first couple of seller financing payments for that final 1/3 on time and am very confident that will continue for the next year and a half until I'm fully paid that final portion.