Can a Pope elevate a non-heretical yet minority opinion to the level of dogma? For example, the empty Hell theory is a relatively recent theological theory that states that Hell is real, but it is empty and will always be empty. There is nothing in this doctrine that conflicts with established church dogma (As far as I know). Nevertheless it is a minority opinion and as I understand it, it faces much opposition.

I'm wondering, is it within the Popes power to take such an obscure and contested opinion and ex cathedra declare it to be a dogma? Or does the Pope first have to consult all the bishops?

I ask this question because I've been thinking about the Arian crisis, which was a situation where the majority of the church denied the trinity and the divinity of Christ (including some Popes and lots of bishops!). In that case it was an ecumenical council which authoritatively declared Trinitarian Orthodoxy, and this was binding, even though at the time almost no one in the church agreed with it and it was a minority view. It took some time for the authoritative decision to permeate the consciousness of the Church and now we have the situation today where the Trinity and the Divinity of Christ are considered cornerstone, foundational doctrines of the faith.

So, similar to the Arian situation, would it be possible for a Pope to elevate a non-heretical yet minority position to dogmatic status, even if the majority of the church disagrees? Again, for a concrete example, think of the Empty Hell theory: It's not heretical, but also isn't accepted by the majority of the Church.
The Arian controversy was dealt with at a time when the doctrine of papal infallibility was not yet established. Although it was an ecumenical council which authoritatively declared Trinitarian Orthodoxy, belief was actually mandated by Emperor Theodosius. I doubt whether, during the fourth century, there was ever a time when almost no one in the Church agreed with Trinitarianism or even that it was definitely a minority view. Nevertheless, I think I understand the scenario you see as possible.

Papal infallibility has only ever been invoked on two occasions, so it would be  an extremely solemn and exceptional circumstance when a future pope would even consider declaring doctrine ex cathedra. Popes have many ways of leading the Church to a position, without the sledgehammer approach that unnecessary use of papal infallibility would involve.

Take the example of human evolution, which Catholics are now permitted to believe in, but which would have been a minority position within the Church hierarchy only a few decades ago. No pope made any doctrinal pronouncement, but gradually, pope by pope, the papacy led the Church towards accepting evolution as fact, until the possibility of evolution of our species is no longer a minority position within the Church ranks.

It is not possible to predict what all future popes will do, but if a pope ever wants to declare a minority position to be infallible doctrine, I am sure he would first set out to convince the cardinals.  With support of the cardinals, he perhaps could speak ex cathedra on the matter, but more likely would call an ecumenical council to determine the matter.