Backstory: I ran a consulting company full time for almost four years and we've always had issues with client proposals. I've used Adobe Indesign, google docs, pdfs and almost everything under the sun to produce professional documents that don't waste my team's time.
The biggest problem was getting feedback from clients. They would constantly reply with "on line four, third sentence, can you change this to that." A lot was lost in the back and forth and it prolonged the client acquisition process.
This is one of my personal favorites because in the end, I found that a truly custom solution will solve this problem.
The Idea: Proposal annotation software for freelancers.
Deep Dive: For a MVP, you could allow freelancers and agencies to upload plain text. Each line of text would be converted to a "commentable." Clients would be able to view the document and click on a sentence to leave a comment. The comment would start a thread to resolve any issues with the wording in the document.
The magical part of this process is that the user can get slack and/or text alerts when their client edits the document. This way you can call or message them immediately to discuss any issues. This closes the gap in the acquisition phase faster.
Additionally you can add the ability to select templates and other visually appealing items to impress the end client and provide more value for your users. These virtually proposal documents would look better than any PDF or google doc and would be easier to produce than anything from InDesign.
Validation: I've built a MVP of this before and I've seen a few startups trying to attack this problem.
Monetization: Similar to other SaaS platforms you would charge a monthly fee for use. You would position each tier with addition features for more money. The lowest tier would target freelancers and only have one user. Each additional tier would add seats and/or features.