If, Hypothetically, Sacred Coins

I don't think gaining a +3 in some stat is as game-changing for most characters as people make it out to be. There are so few heroes who are reliant on a certain IV to attain a good enough power level to make it into tier-20 arena teams, and most of those are the hyper-offensive units who need just a few more points of speed or attack to substantially increase the number of units they can 1hko.

So to answer that question, my guess would be a definite No. You might see a few more of the GHB/TT heroes in arena, but it wouldn't change the vast majority of "meta" team compositions.

But I don't think Sacred Coins are going to be for re-rolling IVs anyway. I do think they will integrate it somehow in the future, but not as a reward from Arena Assault.

Oh yeah, for sure. If only for all those starter hero bonus units/benchwarmers. +atk Brave Sword Alfonse, +spd Sharena, etc would be seen in arena. Not top tier, but they would see use since I still see those starters from time to time.

Sharena - +Spd will help her bulk immensely, though she'll still be outdone by lancers such as Nephenee and Lancina. It would open up opportunities to make her a more dangerous debuffer with something like Threaten Spd though.

Alfonse - +Atk can Run Brave Sword but Chrom does that better. I could see +Def happening on the Defiant Defense Folkvangr set though to help with his bulk.

Anna - +Atk or +Spd will increase her attack power/doubling capability. I see +Atk happening more as it'd definitely improve her Noatun Desperation set a lot and increase her viability as a sweeper, considering her damage as it is is mediocre.

It'd certainly help them, yes, and make them better units to use especially for the weekly arena bonus, but I hardly think it'd hugely shake up the meta to the level of something like the CYL banner, for instance.