This is what I drew up:

Drill
This would have the ability to do damage to water units when the Com is underneath them. It would have a shortly larger range than D-Gun and would have about the damage of a scout but would have a constant rate of fire. It would work by drilling the bottom of sea units.
EMP
This weapon would be able to EMP units for about 3-7 seconds depending on how much Energy a player has. The range would work best at a Jeffy's range IMO. It would have a rate of fire about the same as a Bladewing but would have a projectile similar to the Panther or Zeus. Maybe a bit lighter.
AA
This one's pretty simple. This would act as a proper Anti-Air weapon which would have a slightly shorter range than a defender but a little faster rate of fire than it. Probably same damage as a Samson would work well. This would prove as a nice way of preventing Com naps and people bombing your com on a com ends game.
These are just a few suggestions for attachments but I'm sure there are more creative people out there.
How attachments would be placed into games
There are countless ways to be able to implement these into gameplay so I'll list a few.
- They could be selected in the lobby before the game starts. They would replace the Coms laser or D-Gun depending on the effectiveness. Ofc, laser and D-Gun would still be avalible as a choice.
- They could be bought ingame for an amount of Energy and Metal.
- They could become avalible depending on which level of tech a player has reached.
- They could replace the D-Gun or Laser permanently and have different weapons apply to different factions. For example the EMP could apply to the Core Com and Drill to the Arm Com or whatever.
If different attachments were avalible to Coms, a whole new load of tactics and strategies would open up to the Com units.
For one, the AA attachment would be able to kill transports meaning a lower chance of being com napped.
The Com unit would be able to defend itself under water instead of being completely defenseless.
The EMP could be used in case of being scout raided. Com could even capture the scouts for effective counters.
I'm not going to rabble on about the uses as there would be loads and of course it would be possible to limit players to being able to choose only one of the attachments so many current tactics such as Com naps and scout raids would still apply.
Don't criticize my actual attachment suggestions as these are just my ideas if attachments were implemented but feel free to give your thoughts on having attachments for Coms.