1- being able to bcc or better yet have a gmail extension to log emails sent to prospects and clients is a foundational requirement for us. Be sure to have that asap. Outside of being a CRM, our SF is the knowledge base of all customer interaction.
2- Opportunities and funnel management are critical workflows for us. Being able to define rules on qualification and auto activate / expire a prospect is important.
3- documentation is important. I know there are armies of SF consultants I can tap into if needed. In the absence of that, a strong wiki will be important as you start offering customization
4- speed speed speed. My team much preferred SF classic over lightening because it was significantly faster
5- for us, mobile is overrated. Most work is done on the laptop or desktop. Mobile is more about reading details and freshening up on client ahead of the meeting
Lastly, I’ve dabbled with the idea of offering a light-weight CRM to our clients (we serve a niche industry that has onerous onboarding requirements and the vast majority of clients don’t use an off the shelf CRM). Once you polish this off, if you’re open to chatting about a white labeled solution feel free to connect.
Good luck.