There are boutique investment banks focused on tech startups (at different levels of annual revenue) that specialize in working with founders to help them identify their strategic options. Here are two examples of such firms. Some of the case studies on their websites can help you understand the process better.
http://vistapointadvisors.com/
http://arboradvisors.com/

I'd suggest:

1. Looking at different industry verticals where a truly synchronized viewing of documents is of use
2. Then arrange those in a descending order using a mix of variables such as size of market, perception of pain felt by client, etc.
3. For the verticals that come out among the top 3-4, it might make sense to do another round of customer development interviews to validate your hypotheses around utility to customer

This could potentially point you to a small number of compelling use cases.

I'd be happy to look at (a) what use cases you currently have and (b) brainstorm/be a devil's advocate as you go through the process above.

This Nov 2012 blog post by Bill Gurley of Benchmark Capital is very insightful, and cites some great examples as well:

All Markets Are Not Created Equal: 10 Factors To Consider When Evaluating Digital Marketplaces
http://abovethecrowd.com/2012/11/13/all-markets-are-not-created-equal-10-factors-to-consider-when-evaluating-digital-marketplaces/

Also, see the deck here which takes Bill Gurley's ideas above and translates them into an evaluation framework:
http://www.nirandfar.com/2013/03/making-a-marketplace.html

I'd recommend reading this book to build a working understanding of these issues:

Venture Deals: Be Smarter Than Your Lawyer and Venture Capitalist
http://www.amazon.com/Venture-Deals-Smarter-Lawyer-
Capitalist/dp/1118443616

I'd suggest polishing your in-person pitch and hitting venues where food trucks gather. These events happen weekly in major US cities. You should go one week to observe the customer lining up patterns and then pick an appropriate time the next week (maybe when the food trucks are wrapping up for the day) to approach the food trucks with your pitch.

Here's just one example of what I mean
http://offthegridsf.com/markets/fort-mason-center

If you aren't already doing this, it might be worth using the insights from your podcast on other outlets as well, e.g. sound bites on twitter, key insights on blog posts, and email newsletter which allows individual articles to be shared on social media easily, setting up a profile on clarity.fm for paid business consultations, carefully cultivating relationships with some journalists, bloggers who write articles related to areas of your expertise.

Also, it might be worthwhile surveying your podcast audience to find out what they like/love vs. don't like/hate (think a 2*2 matrix) about your podcast, so you can make the existing audience stickier and potentially expand the audience.

Assuming you have several ideas, and you are trying to figure out which one among them to work on, I'd suggest:

1.Try to sketch the business model for the startup idea (referring to Business Model Generation - Canvas)

2.Get good at doing quick and dirty market sizing of opportunities

3.Check if you are passionate enough about the idea to potentially spend the next 5-8 years of your life in making it successful

Based on a collective evaluation of the above three, you might be able to zero in on an idea to investigate further (e.g. building a prototype, customer development, etc.)

Since you mention that you have achieved "amazing results", I am assuming that you have some sort of a prototype to show to an angel/seed investor. Also, the fact that you managed to get a government grant (which I assume you didn't have to give up any equity for) is a good thing and shows creativity on your part.

That along with a well-crafted pitch about your vision for the product and the reality of the specific problem you are trying to solve might be good enough to start having conversations with some angel/seed investors. Assuming you get some initial commitments, you could then open up a public funding solicitation on AngelList. Please do consult your corporate lawyer for specific do's and don'ts of process of public solicitation.

Also, you can try to limit the portion of this $500K going towards founders' salaries by keeping those at a bare minimum.

Finally, you'll also need to think carefully about the amount you want to raise. Will this $500K be enough to take you to your series A?

Happy to brainstorm more or provide feedback on your pitch deck.

An angel investor is likely to want to:

(a) See that you have thought about the desired profile of the other cofounder, who will complement your skills, and who you'll need to attract to the startup. Even better if you have identified a few potential candidates.

(b) A plan for you going full-time in this venture.

(c) Some kind of feedback from the potential market that you are solving a real problem, and some market sizing analysis (even if it is very rough).

At startup networking events, go prepared with a 30-60 second elevator pitch about your startup to gauge how people react to it, and who shows interest in follow-up discussions.

Happy to talk more on the phone with more details (e.g. whether it is a B2B or B2C venture), so the suggestions could become more specific.

Also, please feel free to check out some of my other answers (on my Clarity profile): e.g. what investors look for in a technical cofounder, etc.

This book can be a good starting point:

Venture Deals: Be Smarter Than Your Lawyer and Venture Capitalist
http://www.amazon.com/Venture-Deals-Smarter-Lawyer-Capitalist/dp/1118443616