I am working for years with a CRM. It became the most important success factor for my company.
So I would write 2 issues - key features and approach
Key features
People (Organizations, Relationships): could be done with taxonomies, cck, ur relationship ...
Invoices (opportunities, offers): maybe with Ubercart intergration - sales pipeline concept should be possible
Contact History (emails, pn, phone calls, sms, visits, ...): maybe with views, heartbeat, activity ...
Tasks: with dates and cck (priorities)
Files: web file manager or similar module
Campaigns, Mailing Lists (newsletter,...): with views, views send, vbo, views rules integration, rules
maybe there are other great modules which I do not know but would integrate very well into a CRM
Approach
As there can be done almost anything by using already existing modules what would be needed is a UI which brings those modules together.
Maybe panels for UI landing pages or storm module. The big potential of that approach would be flexibility and ability to custom tailor ones owns CRM if you are good in views ..... And maybe it is much faster done instead of custom coding. But it shoul be usable without experience as well by just providing default views.
I am not a developer, so I don't know if this approach should be done via modules api or without. I think both are possible.
I am trying this approach at the moment, but I am not good enough and I don't have enough man power. But some things I can already do rudimentary (newsletter e.g. using vbo ...)
So this approach is more a kind of an "installation profile" than a module - or both combined ....?
What do you think?