It would definitely be a good addition if the game could be played entirely in a PvE sense and not forced to do battles as you say. Actually a number of months ago i suggested a “scailing” battle tower where rewards and battles got tougher and tougher over time… @Sixty-Four believes this is where Ludia took ideas for the Campaign thing, ive not played it myself but from what he tells me it sounds exactly like what i suggested all those months ago.
However if its a “do once and forget” thing as you said, that wasnt what i had in mind when i suggested it, as i had hoped it would reset from time to time (weekly, monthly whatever) and be thus an alternative to arena. I suggested such a tower back when i started finding arena less and less fun and was almost solely playing strike towers, and thus why i suggested it.
I might actually login some time when i wanna chat to my alliance and have a look first hand.
But yeah, a fully PvE viable alternative to arena would be nice, perhaps have those PvE battles be counted as matches for alliance missions? that way those who like arena can contribute that way, and those who cant stand arena (yourself, myself and so forth) would have been able to contribute THAT way.
But at the end of the day, we can suggest things all we like, ask for improvements, balancing being done, nerfs (though id rather concrete counters) but in the end Ludia are the only ones who write the code and put new features, so ultimately its on them to heed our advice.