How can we improve WHMCS?

The way marketing campaigns currently work, they are only useful to send emails to NEW customers and not existing customers.

So for example, if you create a campaign to promote an addon to customers who purchased x product, you must define a number of days after client purchased x product to send this campaign.

This will ONLY send that campaign to clients to purchased that product EXACTLY x days ago, nobody else.
So there is no way to retroactively send campaigns to your existing clients.

this is very simple to fix.
It simply needs the rule to be ">=" rather than "-"
i.e. apply on clients who purchased a product/addon >= 30 days

Which would then allow to be applied to existing customers as well.
and of course have the option "has not previously been sent this campaign" so it doesn't resend to the same customers over and over.

You also need to have a way to auto resend a campaign and control how many times a campaign is sent. It is very rare that sending a campaign once is enough, you have to promote somehting multiple times to get a response.

This would be achieve by having a "auto resend max" value with a "delay". So it will continue sending the campaign to customers every "delay" number days until "auto resend max" value is reached or user has unsubscribed.