Hi there! Good news to hear from you. We've just bought WHMCS for use with our DNP, but honestly havn't had the time to configure it and try it very much, yet. Personally I did a quick but didn't get whmcs to talk with dnp properly - not right off, at least. But my first thought was that there might be some issues to deal with regarding this.
Having said that, please consider that we're not fully aware of what options already is in whmcs but I presume it's possible to buy domains, hosting plans and addons from whmcs that is provisioned in dnp?
The part I really would like to see that I'm pretty sure is not there right now, is having whmcs implemented in DNP control panel - is that possible? I don't want customers to log in to whmcs for the invoices/orders and log in to dnp to manage their web. Best thing would be to replace the DNP eCommerce module with whmcs completely (the "My Ecommerce" link from the Account Menu in DNP control panel). Maybe this is all the way around, that dnp has to speak with whmcs's api and not what you're asking for?
On the other part - maybe you should describe your ideas you'd like to perform and then ask people here what they think, pro's and con's and such? I don't know what's possible for you to do at all with that module so it's a bit hard to make a whish list :) But maybe some issues rises soon when I try whmcs some more...