What other modules besides the DM10 would be expandable?
As far as Alesis goes..None..The Pearl Redbox should also be similar but it is basically a re-branded/licensed DM10 with "2.0" firmware allowing you to add soundsets.I am not familiar with any Roland modules that can expand triggers using MIDI but you can always check over at V-Drums.
E-Drum developers didn't go into that direction.."expandable" these days means adding Sounds/Voices..not additional triggers , so that's something to watch for when purchasing and whether or not you can add your own sounds/voices or you have to purchase theirs (specific formats.)
There are some advanced MIDI tricks if a module has a GM section and accepts program changes..has the ability to re-assign MIDI notes or turn off the MIDI notes in the main "Digital" section and re-assign those to the GM section (requires GM mapping) and you would be using the sounds/voices from both sections simultaneously and switch banks/kits using program changes , however you wouldn't have access to the GM Voices for editing and depending on the module... you may run short on polyphony.
expand my kit with an x-hat or some type of variable hi-hat as such, and then a few more cymbals or drums/pads. The Trigger I/O has a limitation with the hi-hat pedal, with a constant 'chick' velocity/volume, doesn't feel realistic, and want to upgrade and expand.
If you are referring to the Pro-X Hi Hat (FSR) ..it is discontinued and single zone.
The Trigger I/O accepts either a single or
dual zone Hi Hat.. so by using a Pro-X you would lose a zone.
You don't mention which Hi Hat pedal you are using.
The Trigger I/O hi-hat control input works with both continuous (variable) and switch-style (open-closed) hi-hat pedals.
I haven't yet tested a FSR Hi hat (Pro-X,STRIKE) with the Trigger I/O.
The DM10 and DM8 modules the Pro-X was designed for had to have a Firmware update in order for it to work properly.
There were a few that tested the Trigger I/O with the Roland VH-12 years ago..but the range is completely different than Alesis.