Maybe we should ditch the Tank Destroyer designation and just focus on the idea of a compact fire support vehicle. In a perfect world, a Leopard 2A5 would show up in the nick of time and send the threat to the other world with a well placed APDSFS dart, but there are precious few Leopard 2 tanks in our inventory, Leopards have mobility restrictions based on size and weight, and there may be times *we* might have to take fire support in by air or sea (Kirkhill's suggestion about the airportability of a Hetzer like vehicle hints at that).
Now a high mobility vehicle like a BV 206/Viking/Bronco has the mobility to get in and out of all kinds of unexpected places, and can be slung under (or if properly prepared, carried inside) a helicopter like a Chinook. The large size allows it to carry extra ammunition and ancillary gear like electro/optical surveillance equipment or an APU which can enhance the ability to engage targets with a missile or a recoilless cannon. The downside is even a Bronco isn't that heavily protected, and once the enemy is aware of your presence (from the launch signature of the missile or the backblast of your recoilless weapon) they will be shooting back. Technical solutions like a "soft launch" missile with a reduced signature or a High/Low pressure recoilless cannon are possible, but have not been developed to the extent that we could engage with a Hellfire like missile or the equivlent to the 120mm WOMBAT recoilless cannon (anything less would just make an enemy tanker angry).
A small protected platform like the hypothetical Hetzer II allows the crew to close up and engage bunkers and field fortifications, fire at exposed enemy infantry with HE, cannister rounds or machine gun fire and if properly situated, ambush enemy AFV's and have a fighting chance of surviving enemy counterfire while escaping. Just as an aside, the newest ROK IFV (K-21) is evidently made of composite materials so has the protection of typical steel IFV's in a 25 ton package, so the Hetzer II could potentially make use of this technology to provide far more protection than might be expected. While a small, high velocity cannon would provide the means to deal with AFV's, it would have less effect on other targets, so a 105mm cannon or howitzer would provide the ability to take on a wider range of targets at the price of less performance against an AFV. A box launcher with Javelin fire and forget ATGMs on the roof could fix that.
So what do we have? A hypothetical vehicle to provide direct fire support in close terrain, potentially capable of being placed alongside light forces to give them more punch and to free up tanks for other roles. Is is doable? OF course; the technical issues are relatively simple, and we have more than a century of AFV experience to draw from. Can it be done? Maybe, if a need presents itself and the Army decides to put resources to the project. Will it be done? Not likely. (Even if I was Generalissimo of the Armed Forces I would preferentially put resources to bulking out the tank fleet and standardizing platforms. You would see an armed version of the Bronco, though)
