I was thinking something similar. Right now I have a "People" folder with a note for each person that I then link to. The title of each note is an @ mention of the person's first name. Then I slowly can build up a history of interactions and information about a person in relation to that note. But a lot of the information in that note is redundant of what's in Contacts, like phone numbers, emails, addresses, and I then have to manage it in two places. Plus, I can't use Siri obviously, based on a note in NotePlan. It'd be better if I could leave the contact's contact data in the record in Contacts, link the @mention to that record in Contacts and then simply use the @ mention to keep building up the relationships. What I'm doing is a bit too complicated for my tastes right now, but it does give me simple CRM.
As long as the mention with contact integration also works without contact integration, I think it would be a great option. :)
For me, this integration would be most valuable after I’ve decided which of my Contacts I want to include in NotePlan. My Contacts db is waaaay too big to be cluttering up an @mention lookup. So far, Apple doesn’t include backlinks from Contacts that could compile content from other places/apps, so, for me, having contact info visible in a Noteplan object, PLUS the content I add there would be most valuable.