A new Jedi (ish, if you know Ahsoka's backstory) character would have worked just as well. There was nothing about this role specific to her character.
Maybe, but it wouldn't have made fans as happy. Also, since Ahsoka is not, technically, a Jedi, having her around does not conflict with Yoda's statement about there not being any more Jedi except Luke and Leia. Though Yoda was not all-knowing.
The idea of Thrawn really makes me wince. I don't like the character, and we already have an Imperial Remnant baddie with a master plan. Tossing Thrawn in there threatenes to water down the Moff Gideon character.
I like Thrawn, but it remains to be seen what role he will play. It is possible that Ahsoka wants to find Thrawn because Ezra Bridger went missing along with him. It is also possible that there will be a conflict between Thrawn and the New Republic that forms the backdrop to ongoing storylines. Mando trying to live on the edges of that conflict could be interesting.
The Beskar steel being resistant, at this point it looks to be totally impervious, to blasters and lightsabers makes the Stormtrooper armor even more comical in comparison. I'd like to see Stormtrooper armor deflect or absorb a shot once in a while, and show that there's a reason to wear it, and Beskar not to be a complete counter to energy weapons. So far Beskar's only drawback is it's rarity.
Mando's pure beskar armor will stop blaster fire, but he still gets beat to shit underneath it. He nearly died at the end of Season 1. And if that spear was lightsaber-proof, his armor should have been also.
As for Empire-era Stormtrooper armor, it is probably more riot control armor than anything else. Useful for stopping improvised weapons and instilling fear in civilians. It is inferior to Republic Commando armor.