One of the cool function of any virtualization management suite is not only to have a nice dashboard showing which VMs performs poorly on which datastore/host, but also to be able to see what will happens if:
The prediction possibilities. If you could predict the future in your life you would probably be milionaire right? You know you can't.
However if you want to predict future in your datacenter? Yes, you can! For example by simulating the increase of the workflows you get the whole picture in 2 weeks, 2 months or more. Pretty awesome.
As an IT guy I usually visit existing datacenters so pure green fields are rare. Usually the datacenter, or the client has already some virtualization already in place. In this case I always use some initial workloads which are then taken into consideration to predict the future expansion.
A client of mine asked me recently on how many new hosts he will need to accommodate X new VMs and the grow for Y VMs a day. No longer guessing. The tools are here for such a tasks and if used correctly they give accurate results.
Did you tried digging through one of those tools or you continue guessing?