Well, I would maybe like this too...
but I think I have things in mind before this :P
For example...
On Reseller Accounts based on SPACE & BANDWIDTH (I think this is what most of us use)
I would like to have options for:
1) limiting amount of users they can have (accounts) non depending on plans, etc.
2) yes, limiting add-on domains will help follow rule (1)
3) and/or limiting the amount (max/min) of space or bandwidth that can be assigned to each account.
that would prevent that if you give 1Gb / 10Gb reseller account for example ...
you dont end with 1000 accounts of 1Mb and 10Mb of bandwidth each ... (get the idea?)
You need to think in what things limit your resources, so that in some way you need to restrict, limit of control on your accounts. (ie. if a user uses 50% of CPU with mysql... is eating resources ... a way of control this would HELP A LOT too) ... you know... a way of limiting how much cpu each user could use with mysql or php, or something like ... I think this is very important, so a user can't kill your server with their forums, it have to have a way you could make that user not use more than 20% cpu (for example) with this... on a per user basis ...
maybe off topic here... lol... but well