Ability to offer round-robin and 'lowest number of accounts' IP allocation to accounts.
Over the years we've seen our fair share of DDoS attacks and, as such, we do our best to evenly load our IP addresses with accounts, or to at least not load them all onto one IP. In the event that we need to null-route an IP which is, unfortunately due to the increasingly malicious state of the internet, becoming more and more common.
To help prevent everybody from going offline in the event of a null route we currently update the "Main Shared IP" in "Basic Setup" in WHM from time to time so that new accounts are created on new IPs and functionally this does the job. There is the side-effect that there are no default virtual hosts for anything but the current 'main shared IP', but I've submitted another feature request for that.
Beyond that, what I would love to see is the ability to specify a list of IP addresses instead of a 'main shared IP' that can be used whenever the reseller doesn't have a specific shared IP or the account is being created as root.
This could be similar to IP delegation to a reseller in that it could list IPs and let you check them off as needed.
Additionally an option to choose how it allocates sites to this IP would be great. Say you have 0.0.0.1 through 0.0.0.5 checked. You could choose 'lowest population' where it looks at all 5 IPs, and places the account on the IP with the least number of accounts assigned. You could also do 'round robin' where it adds one account at a time to each IP and moves to the next. I.e. Account1<-0.0.0.1, Account2<-0.0.0.2, Account5<-0.0.0.5, Account6<-0.0.0.1, etc.
The default option should be to use a single main IP as it is now, but if a provider does wish to spread accounts across a few IPs in an automated fashion - they should have the ability to do so in my opinion.