I dont really want to derail this with another debate about 3E and balance, but my experience was I didnt really encounter huge issues with the game until people started bringing in builds based around many of the feats and prestige classes in the complete books. You are certainly entitled to believe 3E is a bad base, but plenty of people have been using it now for thirteen years and still love it. I myself am just starting up a new 3.5 campaign after a few years away. Re-reading the phb and dmg I have to say I am a lot more impressed this time around, and I think it is actually a incredibley well done system in many respects. If you use the complete system you tend to run into fewer problems in my opinion (I still think there are some issues with the magic, but nothing I cant deal with).
See, the thing is, I compare 3e and 4e here and think about what I learn from that. 3e has a much less regular rules structure. Class design in particular is all over the place. I don't seek to get into debates about past editions either, but CLEARLY without any real effort or often even meaning to a player can make a level 6 Druid or Cleric that is just insane, way more capable than most of the other characters at that level. Clearly this lead to plenty of issues, but that's not my main point. The issue is that everything that followed from that initial point in 3e's PHB inherited those problems. It all had to be built around the fact that you could MC and cherry pick ANYTHING, and that class mechanics were all over the map, etc. That may have been a wonderful game for you, that's fine. It was CLEARLY not a wonderful game for supplement developers. The whole history of 3e illustrates that. They spent the next 2-3 years wrestling with these issues, with 3e's supplements generating an every increasingly unstable mass of rules, until finally there was obviously an awareness that the whole system was just collapsing under its own weight so to speak. 3.5 was the result of that, but obviously nothing short of a total rewrite was going to deal with those issues.
And then we have 4e. There are things in the 4e PHB that don't quite work perfectly. There were some 'broken' powers and combos and etc, but everything was built on a platform that allowed designers to clearly understand what they were doing. If you made a new class you HAD to define how it fit functionally into the game by defining roles and a power source. You had clear examples of class mechanics that supported various thematic and mechanical goals. There was no vast concern you needed to have IN GENERAL with the interaction of things with the rest of the game because the fact that your mechanics were designed around a template allowed the core rules to regulate that. As a result, even though 4e released a vast blizzard of material over a SHORT period of time (consider, 4e really has released relatively little in the way of player options in hardcover since 2010, so most of it was done in 2 years), they were able to keep the game WORKING. You can combine all that material and play a game with literally no house rules or material restrictions of any kind and it will work as intended from levels 1-30. 3e is not even close to that.
So then when we consider 5e, and I see a game that is once again built around a chaotic jumble of class mechanics, and presumably if we are to believe comments made here and there 3e-style MCing, then how would anyone expect that it won't crumble under its own weight just like 3e did? In fact the same fundamental issues exist, they are inherent in the philosophy of rules design used in 3e. We can even go back to 2e and see how it suffered the same issue where all the options and things created a monstrous swamp of rules that the DM had to constantly prune and rework if he expected them to produce a coherent game that wasn't all over the place. I mean I don't have a problem with the possibility existing to go out and just do wild things with the system when and if you want to. I just don't see how a core system that provides the solid results that we got from 4e, where it worked even after 20 supplements, can hurt things. I mean nothing stops one from just grafting any sort of mechanical superstructure on top of something very similar to 4e to produce whatever results you want. How hard is it to drop on Vancian casting and the 2e spell list? Old style fighters ala 1e? Crud you can most certainly import the class mechanics of any class from AD&D right into 4e with no more than slight mechanical rewiring. Of course it isn't going to be exactly a 4e style class, but that would be the point...
It seems to me that the way things are developing is bass-ackwards as we say in these parts. The stuff that should be DDN's OPTIONS are the core and (well apparently not) the core stuff is options!! All I can do is roll my eyes, and that takes me right back to 3e where the day I cracked the PHB I was rolling my eyes...