Unit Wonders?

I just had a silly thought. What if there were units you could construct that functioned like National Wonders - you could only build one, ever, but it is incredibly powerful and useful, but requires strategy to make the best use of.

As an initial example, the Trojan Horse. Every civilization can build exactly one Trojan Horse, and how it works is that it disguises itself as a Caravan, only you control it directly, and once it's in a city, upon your command you can order it to release and do massive damage, an immediate declaration of war, at which point you rush in and take it handily. Maybe it would destroy the unit, maybe it wouldn't, but it's definitely a devastating superweapon for the Classical era. However, if the enemy catches on to you sending in a Trojan Horse, they may just go and attack it, killing it instantly and sending all that production and opportunity down the drain.

Similar in case, each Ideology would get a Unit Wonder or Great Unit or whatever you call it. First one that comes to mind is the infamous Landkreuzer for Autocracy, the ridiculous impractical supertank that sadly never made it to reality UNTIL NOW. It'd be a crazy powerful tank that can get in standoffs with cities, and sweep hapless infantry away like they were nothing. Of course, any given enemy is going to prioritize destroying this thing over all else, and as with any armor unit, it's not going to do well against AT guns. Maybe, just maybe it could barely survive one round of being pelted by five ATs, but good luck getting away. Order is a bit trickier, and the placeholder idea I have for it is giving them the maintenance-free Foreign Legion instead, one in every city. FLs are still pretty crazy, and for a wide empire, getting fifteen or twenty of them for free is crazy powerful and something you could well utilize as the backbone of your invasion, especially if you upgrade them to infantry (or as Brazil, into Pracinhas :O). And of course, for Freedom, they get exclusive access to the Atomic Bomb, though later anyone can build Nuclear Missiles. However, the Atomic Bomb is a one-shot. It's the last-resort weapon for ending a dangerous war as quickly as possible, and firing it means the target civ is heavily crippled, and will almost certainly beg for peace shortly thereafter.

Whatever do you think? Would this be a good idea, prospective weapons of mass destruction throughout time and history that can bring great conquest prowess to those who can use them effectively, and embarrassment throughout the ages for those who squander their expensive weapon, thinking their ridiculous and impractical plan would actually work? ^^;

There were civ 4 mods that did stuff like this. Some units could even only be built once ever. They were pretty fun and interesting to play with.
I'd actually like to see it again.
I'll come up with ideas later.

Intriguing, but I wouldn't make it too sci-fi or too mythical. Just something that was ahead of its time, and not necessarily constrained to one civilization.

I was thinking of a bombard in the Medieval era. A railway gun in the Modern era. Of course, these are just siege weapons...

Civ 4's lends of revolution mod had lots of these units. For example, the 300 Spartans, which was a spearman with extra strength and a bonus vs melee on top of its bonus vs mounted.
Some other (admittedly somewhat tipsy) ideas are:
U-Boat: same as a submarine, except higher combat and ranged combat strength, double gold from pillaging naval trade routes.
Crusader: stronger knight that requires catholocism and gets a bonus against units belonging to a civilization of another religion.