With the past few champions released, there has been a lack of champions who scale off of both AD and AP. This is most likely due to it being a balance issue, since you have to balance around both builds. Shaco is one of the few champions Riot has historically tried to keep effective whether he chooses to build AD or AP. Over the past year though Riot has started to stray away from keeping magic damage in his kit. The rework took away the 20% bonus damage that his passive had; This would increase the damage of both his boxes and his Shiv(E), increasing the damage on his E from 100% AP ratio to 120% AP ratio when hit from behind. Before the rework this was one of the biggest reasons AP Shaco was viable. The rework took most of the damage away from Shaco's E and put it onto his Q, which was later changed to his passive due to AP Shaco jungle being too strong post rework. Shaco's E was also changed recently from magic to physical. This was a nice buff to Ad Shaco, but took a lot of the magic damage away from AP Shaco's kit. Shaco is one of the many champions on the list to be given a full rework.
With the preseason starting, and adaptive being a big part of it, I propose that Shaco becomes one of the first champions to be given adaptive abilities. Both his passive and E should become adaptive. I think Shaco should be the first, since Riot has balanced him around the varied damage in his kit. This would allow for Riot to test how adaptive abilities function in the current meta. There are both pros and cons to giving champions adaptive abilities, so I don't think all champions should get them.
The biggest pro, would allow for more innovative builds. Champions like Tryndamere, for example, would have his E become adaptive. Ap Tryndamere was a fun way to play him before his Q's healing was reduced. He still has the ratio on his E, but It is a lacking build due to it not dealing magic damage while building magic items. For instance, You would have to get a last whisper instead of a void staff. Giving it adaptive damage would allow for his E to do the magic damage that he wants, allowing him to continue his magic damage build. This would also give the ability to diversify team comps when having a full AD or AP team. With this, there are cons to consider.
The biggest con, that I can see, would be that it's hard for opponents to itemize against the adaptive champion. Not knowing if they will be going AD or AP. A tank going into a matchup would see a Tryndamere, and instantly think armor items. He would start his build by rushing Ninja Tabi, and the Tryndamere would come back with a statik shiv. This would make you think he's still going Tryndamere's usual crit build. So the tank player continues building armor, his sunfire cape next, but Tryndamere comes back with a blasting wand. Now his E will be doing magic damage, causing the value in the armor, the tank player bought, to be reduced while in laning phase. If Trynd is able to build adaptively it is harder to itemize against him, but this also prevents him from becoming useless as the game goes on.
Giving champions adaptive abilities would give more flexibility to champions who have only one possible build path. It would add diversity to the roster of champions, increase flex picks, spicing up the competitive aspect of the game, and granting more usability to many of the 2 dimensional characters currently out there. As a champion, Shaco has been traditionally defined by the versatility in his build path, and exemplifies many of the characteristics I have stated thus far. This is why I feel Shaco should be the test subject for adaptive abilities.
tl;dr Some champions should get adaptive abilities. This would add competitive aspects to the game and open up build paths to diversify 2 dimensional characters. Could be hard to itemize against. Shaco should be the first since Riot has tried to balance around his AD and AP builds, but have hurt one while trying to fix the other..