Now that 500x is out, they need to allow 10 FX blocks as there is 20% more DSP. I currently use POD HD Pro only for effects so without amps and cabs more FX should be a no brainer. I run dual tones all the time so tend to add FX in pairs. Tron up left and tron down right, fuzz pi left and jet fuzz right. leslies left and pitch vibrtao right etc etc etc. Since simple things like volume and loop take up a slot this would also help that issue.
More thought to acoustic stuff like preamps. Having it part of dream rig with JTVs and not having enough tools for acoustic sounds is a bit whacky.
JCM 900 Clean
In fact with just those two I would be thinking about a 2nd HD Pro and actually use the amp models.
I really wish they would do a POD HD 1000 Pro with two of the newer DSPs so people could really create more complex patches without DSP issues. Charge $1,000 or whatever is needed.