How about something like this:
You could fit a whole scuba tank in that.
In all seriousness, I doubt that this is the type of response you're actually looking for, but the guidelines in your initial post leaves you open to just such responses. You currently list six criteria:
1) Gun must be able to be converted to HPA.
2) "something you don't see everyday"
3) "something modular"
4) "something comfortable"
5) "looks badass"
6) "gets the job done"
The problem is that we need more information to truly offer quality suggestions, I will go through some questions for each criteria:
1) Do you want a custom HPA job, wherein you are creating your own conversion parts? Or do you want to convert a AEG to HPA using one of the retail conversion kits? You must keep in mind that the retail kits are limited to particular gearbox and gun configurations, at least without at least some modification.
2) If might help us to tell us what you're not looking for, or guns that would not qualify.
3) the M16/M4 family is probably the most modular platform available, but based on criteria #2 I am assuming that M4's are not what you're looking for. Again, though, more direction on #2 would be helpful. Otherwise you can slap a rail on just about anything nowadays, just depends on how sacrilegious you want to get.
4) This is a bit subjective, and it would help if you told us what you find comfortable or give us an idea of your physical size to help us. For example, a guy who is skinny and 5'4" isn't going to find a full metal M60 all that comfortable.
5) This is so subjective that it doesn't really add anything to the conversation.
6) What job are you looking to complete? Will this be a plinker for fun? a regular everyday use skirmish gun? What role will it serve for you?