There's a Librarian and Chaplain Dreadnought out there. So why not a Techmarine one?
First off: there's an argument for it and against it in terms of story:
For:
Techmarines, having a strong association with the Priesthood of Mars, may well view becoming a Dreadnought as a step bringing them closer to the Omnissiah. Since it seems every techpriest's goal in life is to be more machine than man why not extend this to Techmarines?
Against it:
Since Techmarines are basically the mechanics and armorers of any Space Marine chapter one would imagine their daily tasks would be much more difficult to perform if they were now a Dreadnought. And since a Techmarine occupies a very special status, which is also affected by his association with the Adpetus Mechanicus, it is unlikely one would be able to become a dreadnought. After all, it seems to become a dreadnought you have to be a warrior of such skill that you're talents must not be allowed to die. I assume the person who determines who is allowed this honor is the chapter master. And he may well regard Techmarines unworthy of the honor due to that special status they possess.
But then this raises the question: Is not a techmarine's skill MORE valuable to a chapter than any warriors? Without him the chapter is going to be in a lot of trouble. And do not Techmarines fight just as fiercely as any warrior, especially when it comes to recovering vehicles? Clearly a Techmarine has just as valuable combat skills on top of his vast knowledge.
Technological point: Techmarines replaces their limbs with all manner of gizmos. They also have the servo harnesses. And we can assume they too can create those mechanical tentacles the techpriests use. So if you think about it even in Dreadnought form he'd be able to perform repairs to things. Of course on the battlefield he's now a bigger target. And surely a huge machine stomping around in the workshops onboard Space Marine ships isn't practical. He likely still could contrail servitors in his Dreadnought state as well to assist him. Still, it's an interesting idea to explore.
So what would one look like?
I'd imagine he'd be the baddest of them all in Dreadnought terms. His armor would be stronger than the other variants. As would his weaponry. A conversion beamer would likely be one potential weapon carried by them. Or maybe a modified Thunderfire cannon. In addition to all the other weapons available. I assume Dreadnought are capable of swapping out arms before battles so the possibilities are endless.
But I would imagine he would also have a lot of demolition and repair equipment built into him. After all, who knows better how to take out a bunker or tank than a Techmarine? So he likely would have a dreadnought sized servo harness mounted to his hull. Or perhaps his close combat weapon is some horrific thing consisting of plasma cutters, saws, pneumatic hammers, and other gadgets.
What kind of abilities would he get in game terms? A boost to destroy tanks? Or perhaps some enhanced repair capability?
Just for the sake of fun I'd like to build a Techmarine Dreadnought.