There is a lot of under-representation in our fiction, and in AV fiction often when a member of a marginalised group is depicted the part will be played by a member of a dominant group. Women are usually no longer played by men, although roles are limited. People of colour are usually no longer played by white people (but still too often), although roles are limited and frequent opportunities are taken to white-wash works and replace characters of colour with white characters. Trans characters are nearly always played by cis actors - almost always a trans woman is depicted and if she is an object of humour or ridicule she is played by a cis man; if we are supposed to feel sympathy for her she is played by a cis woman. Disabled characters are played by abled actors faking a disability in most cases.
Consequently many people advocate for better representation, like the outcry against the Avatar movie being transformed from one inhabited by Asian characters into one where a small band of white heroes saves the world (the one being directed by M. Night Shyamalan, not the one by James Cameron, which looks to be Super Space Colonialism anyway), or that people with disabilities and trans people should be cast to play the roles which represent them. I've also seen some backlash against this from among the people in question. At least wert trans people playing trans roles, people have argued that if we have that happening the actors will get typecast as 'trans actors' and their careers will be stalled due to being restricted to trans roles in an industry where there nearly are no trans roles. I would be unsurprised if there were similar protests elsewhere.
I don't think that should be the case though. Would it actually happen? I suppose it might, although given the current situation where cis actors play cis roles and cis actors play trans roles, and so on, it still seems like an improvement over no representation.
What I would like to see, in addition to more representative casting for existing roles is more diverse casting for roles which are not specifically marked as 'minority parts'. I am not inclined to agree that, for example, the aspirational goal for trans actors should be to play cis roles. I don't see any problem with such casting, but nor do I see a reason roles shouldn't accommodate the actors cast for them. It happens a lot in response to protests against things like white-washing of characters of colour, so why not turn it around?
That is, they tend to say "This character doesn't need to be Asian (or disabled, or female, or whatever), the story has universal appeal, so why can't ey be played by a white man?". (and again, often when people are arguing for the universality of a character or story's appeal seems to be when they are reaching for a straight, white, etc. man to represent this universality) So why not the other way round? We don't call straight men typecast if they only play straight men. Nor white men, nor abled men, nor cis men... but most roles are written for them. Unless the story actually depends on the character being one of those things, what would be wrong with casting someone else and tweaking the role to fit? Explicitly not meant to be about turning characters into gimmicks, because being not a straight white abled cis man isn't actually a gimmick, it's being also a normal kind of person who happens to not be that kind, and there's plenty of variety everywhere. Very rarely does the character actually need to be that man, so it is suspicious ey usually is.
Since I don't believe any group of people other than 'talented and / or skilled actors' has a monopoly on better acting ability than others, this leaves the conclusion that there are other factors than 'ability to play roles' involved in why most people we see in films and television aren't women, a third of them don't have disabilities, less than one in ten is other than straight, or why most of all of these people are white. If we were casting strictly to acting ability and weren't so biased in our conceptualisations of what people ordinarily are, I think our working actor demographics would be very different.
And of course, we aren't yet in a position where changing things in the other direction would be fair. It is after all the problem at hand.