Personally, I think ballistas need to be redone, as they are currently useless. Who would ever build a ballista that targets themselves? No matter what else is done with them in terms of what they will or will not target, this has to go.

For that matter in coop games, ballistae are equally useless if they shoot other players as well. Ballistae need to have a toggle switch for shooting players - "Don't shoot me" and "don't shoot any player", or just have that controlled by whether the PvP setting is on for the session.

Besides this, the trophy system while superficially seeming reasonable is also not great. Having only the "shoot everything (including me)" and the "shoot only this one creature" is not useful. Being able to load multiple trophies to allow shooting a range of targets might be a bit better, but not much. Given trophies are somewhat rare drops it will take a good while before you collect enough Gjall trophies to be able to set up multiple ballistae to defend your base from them.

I think a more obvious option is two have another toggle for "shoot flyers only", "shoot ground mobs only" and "shoot everything". This in combination with the "don't shoot me"/"don't shoot any player" would give a more acceptable range of options that might be easier to code than some sort of targeting array that would list every mob with a "Y/N" option.

Personally, given their propensity to devastate my Jotun Puff crops my main use for ballistae might be field defense, IF the dang things would stop shooting me.