Yo bro!
I just finished watching the gameplay trailer myself! Serendipity ftw!
At the moment we see Geralt using the same shit over and over again because that's the armour and weapons that PR wants to iconify with TW3. Standard marketing stuff. It's easier to sell a product in the West using a prominent character with a consistent look. Randomizing him every two seconds could potentially confuse people. Fun fact: Trying to sell a product using a particular tone or atmosphere in the promotional material is easier to do in Japan. Having a strong character on the box art isn't important as it is in the West.
We will be able to get and even craft new weapons in TW3, the same goes for armours. We don't know anything conclusive about changing the character himself. As in hairstyles, beards, tattoos, etc. Though I don't think it would be a stretch to assume that we'll get a chance to change those things as well. At least I hope so. But there's gonna be a lot of different weapons and armours so that's awesome. I hope they're utilizing Dark Souls approach of equipment upgrades. I hate having to switch to a "better" armour if the aesthetics of it doesn't appeal to me. It's a false dichotomy as well. We CAN have both. Developers need adapt FromSofts ingenious way of upgrading the stuff we want to use. It's a great way of making ALL weapons and armours relevant, even in the end game.