Billing & Support Tickets in solusvm client side UI.
Let users see their balance and invoices and pay them from solusvm UI instead of going to whmcs.
Let them open and reply to tickets from solusvm UI as well.
This can be done by utilizing WHMCS api, so instead of integrating solusvm into whmcs you can integrate whmcs into solusvm.
As things are now there is no real reason in using solusvm with whmcs as users will simple get a simplified UI made for whmcs addon and wont take advantage of the modern looking vultr like UI of solusvm.
And having users go back and forth from solus to whmcs and from whmcs to solus is very tiring and beats the purpose of a modern looking cloud vm panel like this.