If you're looking for a simple, lightweight structure for laying out a development project and making sure the goals, deliverables, process, stakeholders, etc. are all in alignment, this template is one of the best resources I've seen:
https://almanac.io/docs/template-project-plan-5a1ff5bf1f8c015dc74408ffe066887a
If you are searching for an application template, and up-to-date advice on all things Y-Combinator look no further than these articles! These great assets were created by a Harvard MBA student through several interviews conducted with previous Y combinator applicants and participants.
Template: Y Combinator Application
Spotlight: All-things Y Combinator
Interview: What makes a Y Combinator application successful?