An auto attack Dependant Marksman (Twitch, Vayne, Ashe) who would have a mana/resource-heavy toggle ultimate that turned all of his physical damage from his basic attacks into magic damage. So Late game you'd see a bunch of purple crits everywhere.
Would a champion with a kit like this be difficult to balance?
Magic crits? Lawdy, Lawdy... That'd have to be some heavily-considered content.
If the only thing the ultimate does is act as a toggle between physical and magical damage, it'd be a bit of a weak steroid that's okay against general builds (MR itemization is lacking a bit), but wouldn't be spectacular by any means.
Why not just go full out and have such a champion have a passive that converts all physical damage dealt into magical damage, in addition to converting armor penetration into magic penetration? You'd have to make some compensations (eg. lower base AD, AD/level, ~90% crit damage, interaction with Sorc Shoes, etc.) but it'd otherwise provide an interesting niche.
So what you're saying is you want a champion with all of the strengths of a hybrid champion (Impossible to build to defend against) with none of the weaknesses (No late-game scaling).
You would have to consider how the magic damage scaled pretty carefully, cuz you can get upwards of 1000AP with a soulstealer and the right big AP items, you're not gonna get anywhere near that much with AD items, otherwise its a cool idea for a DPS ap char other than kayle.
Yes
This is actually a really cool idea! I would like to see a champ like this, except maybe instead of a toggle it could be an active ability that lets his next shot deal the damage in the form of magic damage instead of normal attack damage. The active would be on a short cool down (7-9 seconds) and would cost alot of mana or even increasing mana (similar to 
or 
ult)
It'd be another teemo as far as I'm concerned..
