I'm not sure we have come to consensious on the stats on a superweapon. Or do we need common stats? But the point is we need a common:
*Damage
*Weapon Regeneration time
*Splash radius value
*Range
*And hopefully a somewhat similar amount of time it would take to make a unit that could shoot this weapon (so that, say, it doesn't take 10 minutes to make it for Tech and only seven minutes for Magic).
*We also need to decide whether a unit that shoots the superweapon can move or not, because this could give huge advantages if you could move to get within range, as opposed to having to build a unit there (and the various risks involved with that).
I'd use Command&Conquer:Generals as a guideline for this. Each faction constructs a superweapon capable of wiping out a fairly large portion of the enemy's base. China builds a tactical nuke, the US builds a particle cannon, and the GLA builds a platform for launching storms of anthrax scuds. If you haven't played it, I recommend getting a demo of it. It provides some pretty awesome superweapon examples. The key is that each of the superweapons act differently, but none are really better than the others. The nuke will destroy anything it directly hits and does a pretty decent splash, but leaves radiation in a big area that gradually kills everyone in it. The particle cannon does massive damage, but can only affect a few buildings per use. The scud storm has a massive area of effect and kills any troops in the area with the anthrax, but doesn't do a fantastic amount of damage overall, so most or all of the buildings in the area will remain standing.
Given this, my ideas are as follows.
Tech:
Bombard: lots of damage, small splash. Although its area of effect isn't all that big, anything within it is completely destroyed.
Magic:
(insert awesome name here): a lot less damage, but much bigger splash. It shouldn't do enough to completely destroy any enemy buildings, but all but their most powerful units within a pretty big area should be killed.
Flying units should probably be immune to superweapons.
Range/movement: They should be too big to move, but have enough range to hit anywhere on the map. The bombard used to take Constantinople had range well over a mile, but took about a week just to set up in a position. Besides, even if you gave it a movement value, it would probably be too big to fit through the numerous trees and river crossings on any map, like the big bugs in the SST mod.
Cost/construction/recharge time: Once again, I'd use C&C:Generals as a good reference for this. I haven't played in a while and my copy of it is at my folks' house, so I can't really give you a good idea of the time frames, but I remember the recharge times for the superweapons were at least a couple minutes each. Maybe about four, but I really don't know. I'd recommend downloading a demo of the game and give it a shot.