New Antigua, w/ Twist.

Visuals: At the four star level, it'll be the same color as the antigua, but with snipe on it. At the five star level, two snipes.
Stat bars: It'll have mostly the same stats as the Sentenza and Argent Peacemaker, but with either piercing or normal. Same damage stat, slightly better speed stat, same six bullets.
Charge attack: Same as the others. Six bullets, then a giant snipe. Realistically, it won't be used much, but niether are the current ones and that's apparently balanced enough to not be changed.
The twist: It has a natural negative ASI bonus of high. (Notice this is seperate from simply being slower.) Also, for every ten hits that damage something without missing, it gains a positive ASI bonus of low. The bonuses gained are reset when a bullet doesn't deal damage to anything, and we'd need some sort of *ding* noise or something to notify knights of the reset.
Since Antiguas don't tend to just randomly hit targets they weren't aiming for, you'll have to make sure that your bullets will actually have a target to hit instead of just spraying and praying. This means more skill equals more effective.

It could be a 5* piercing gun that doesn't root you in place.

Overly complicated mechanic! I'd rather just have a plain 5* piercing Antigua line with no extra mechanics. Some extra damage to beasts perhaps. Make it a hunting gun. That'd be nice...

For the exact same reason of why there can't be a third SS line, NO.
It's useless. Why would I want to use it? I'd rather grab an antigua with an UV/swiftstrike/trinkets/gunslinger and have permanent ASI without any negatives. Even a plain antigua is good enough, they don't need ASI that much.