Using Mythical Pokemon has never been done before in a VGC-esque format, so I'd imagine most people don't have a clear idea of what to use yet. I'll talk about the thoughts I've had about it.
1. What Legendary/Mythical to Choose
One way to think about it is to see what of the "Ubers" can fill an existing role better. for example, Mewtwo would be a ridiculously powerful Psychic Spammer next to Indeedee-F as support. It's stats are better than Alakazam in almost every way (except 5 points of Special Defense). Melmetal is another example, it would be incredibly strong as a Trick Room + Bulldoze sweeper. It gets max Steel and Max Ground to boost its defenses as well, making it very tanky in addition to powerful.
As to new roles, Zacian is a super powerful Pokemon that can blow giant holes in a team due to its high offensive stats and intrepid sword. Follow me support and a way to prevent trick room would be very helpful in allowing it to sweep.
Marshadow seems like it could be very interesting in a Dynamax format with Spectral thief. Copying an opponent's boosts from max moves late game should prove very powerful.
Magearna also could be very powerful because of its ability, Soul-Heart, which seems like Moxie but is different because it activates when your partners faint too! You could run lots of gimmicks with it, the ones that come to my mind being choice band Self Destruct Snorlax and Speed control (Whimsicott or Dusclops) plus memento. You could also just pair it with a strong Dynamaxer (Porygon-Z, maybe?) and allow it to pick up some KOs.
2. What else to Bring
This is a lot more general, but the vast majority of the "Ubers" are weak to ghost or ground, meaning Sand Teams (Mostly Excadrill) and Dragapult are both good choices to consider when building. Cinderace could be very strong as well, with fire, dark, fighting and flying coverage hitting a lot too. Especially for powerful sweepers like Zacian and Eternatus, lots of support through redirection, intimidate/snarl, (Incineroar is great here as always), screens and speed control is valuable. As to EV spreads, unless you're really worried about surviving a specific attack or OHKOing a certain Pokemon, I don't expect them to change that drastically.
I hope this helps!
