Diana Needs Changes
As a Diana main I feel like Riot has been ignoring her for too long. Its been ages since she was anything resembling meta. If you really know what you're doing, you can still make her work mid or jg, but she's tier 3 at best in midlane and tier garbage in the jungle. Yes, they buffed her e a little a few patches ago, but comon, that does nothing to solve her problems. Every meta assassin in league has some way of at least attempting to get out after they've blown someone up. Katarina has her shunpo, leblanc has her w, kha'zix has his e reset, rengar has his empowered w, etc. Diana isn't weak because she can't blow someone up, she's weak because that's all she can do. Once she goes in, she dies, and its up to the player to try to do as much damage as they can before that. A possible solution I've come up with is to allow Diana to ult to allied wards. This way, she can lay down a ward before jumping in, then if she gets her ult reset, she can choose to either ult an enemy again or ult out of the fight. I don't believe this is broken because her ult doesn't have that long of range, wards have long cooldowns, her ultimate has a ~15 sec cd, and her ult can easily be interrupted by any hard cc or cc that displaces. Other than that, Diana is a perfectly viable assassin. Her burst is fine, albeit slightly more difficult to execute than some assassins, post-6 her mobility is acceptable, and she sucks if she's behind just like any other assassin.
. This is a rather hard concept to design correctly. One because ap items grant high offensive power compared to ad items (think about it, a full build ad carry has around 300 to 450 ad late game vs a full build ap carry has around 600 to 900 ap late game). That would mean that they will have to tone down Diana's ap scaling on her abilities relative to her ad counterparts. But not so fast, ad items grant attack speed, life steal, crit chance, and other on hit effects to compensate for the higher magnitude of power ap items grant. So what is the problem? The problem is that she pretty much will be outclassed by every other champion in this role. What is the purpose of having a melee tanky ap fighter if you can just do the same job. perhaps even better, with an ad diver like Xin Zhao or Irelia, or even a hybrid diver like 
. They will most likely sustain themselves better, and deal more consistent damage since auto attacks scale off ad, and only auto attack based ap items are 
. But wait, isn't Diana an ability reliant caster, so shouldn't she be compared to caster ad divers? Hmm, lets see we got 
. Pantheon falls off late game hard (needs a rework himself), Lee Sin falls off somewhat too but, along with Vi and Wukong, offers a much more valuable utility in team fights than Diana could ever offer. Hmm...then perhaps the team lacks any ap damage and tankiness, so having an ap bruiser would be nice? Then go tanky mages like 
or 
who can feel just as tanky and not have to risk getting in melee range, or how about going an ap tank who, although it may offer less ap damage (not really in this current condition), offers more cc and front lining power. The thing is that Diana's kit is very flawed, as typical with many archaic champions. You can say the same thing about many of these bruisers. Go glass cannon, and they are basically assassins. Same thing with tanks. Go full glass cannon, and they are also assassins. In the case of Diana, full ap build became standard because her kit successfully functioned better with it than bruiser ap Diana. Diana's full combo rotation is very instant and deadly and unlike other bruisers, back in the day, having two instant dashes was consider highly mobile. This linear pattern of exhausting the same combo instantly and repeating the same process is very typical for many archaic champions who are also in need of reworks themselves. The assassin update focused on reducing the sudden burst of many assassins, allowing a window of time for enemies to retaliate. In return, the assassins were also granted more depth into their kit that allows them to counterplay the enemy if executed correctly. Before many assassins were just very linear in their style of playing. Just dive in at the right time, and exhaust your full combo, and keep repeating this process. The same issue is going on with divers themselves. They jump in, and do damage in the same rotation, but unlike assassins, they have no way of disengaging instantly (that is intended since they are supposed to be durable in exchange). Divers really are lacking depth since their kits are so simple and linear. If you go a full ad Xin Zhao you can probably kill enemy adc in team fight if you dive at the correct time, but you'll probably be blown up instantly too, as opposed to a full ad Zed who can kill the enemy adc as well, but walk out completely unharmed. This very idea is what Riot uses to discourage champions from becoming viable in roles they were never meant to fulfill. Why else did Riot do something about tank 
. They were meant to be assassins, but were trespassing into bruiser territory, in fact, outclassing them with their better mobility. I understand your point on wanting assassin Diana to be the standard, but that is not Riot's vision. They want to bring a champion that is more groundbreaking and unique. Think about it, Diana has a passive that requires her to do use some auto attacks, in order to proc, and so she would need durability which she gains from her shield. But all this is not used completely efficiently if she doesn't build full ap, like I said before, because in bruiser build she will get outclassed by true divers. Because she wasn't updated in the assassin rework, she was also left with her linear kit, while other assassins became more up to date with the styles of playing today, and thus, more viable.