Why is that a problem?
How is this any different from using or not using feats, or multiclassing? Why would it EVER be an issue, as if some players didn't communicate with their DM before chargen??
Why are you questioning a basic reply as if there somehow is something unreasonable with it?
I think I probably just don't understand your examples:
- adding bloodlines or wild talents
- racial levels*
- expanded backgrounds*: your choice of background giving access to unique abilities or perhaps starting gear
- adding a new class framework ("advanced player's handbook") where you don't get bonuses automatically but instead get to choose at each level. Essentially redoing classes to feature fewer automatic class features.
- prestige classes
But your aim here is clear:
My aim is for "basic" and "advanced" characters to coexist.
And that's a good aim, in fact it's a general 5e aim.
So probably I am thinking of something different, when I read your list.
For example, "bloodlines" makes me think of something that each PC can pick at 1st level and gets some benefits (some immediately, maybe some others later). If a PC
doesn't pick a bloodline, he's straight worse than a PC with a bloodline.
Similarly, "Expanded backgrounds" sounded to me like adding more benefits to existing backgrounds at no cost. That forces everyone in the same group to use expanded backgrounds, or be worse than characters that use them.
To me that's a problem, and that's why I say that WotC won't probably do it: it means that PCs using an advanced module aren't compatible with PCs using only the core frameworks. Feats don't create incompatibility because anyone not wanting feats gets something else instead (ASI).
On the other hand... "Racial levels" originally made me think that the choice of race gives you also some benefits at later levels. That would mean that characters using racial levels would be better than characters not using them. But now that I think about it, maybe you meant actual
levels that would work as in multiclassing? In that case, you could take your next level in your race
instead of taking the next level in your class. And that would be OK, because both choices would be equal.
And just as well, prestige classes wouldn't be a problem either (assuming they work more or less as in 3e), as a player could happily ignore them and just level up in their main class, without being effectively inferior to other PCs.
A framework for not getting bonuses automatically but instead cherrypick
could definitely be made to avoid the incompatiblity problem. In this case I am still skeptic they would ever do it, but for another reason, namely as I already mentioned that it probably needs too much design and playtest work compared to how many people are interested, especially considering how much the target audience is demanding in terms of balance and robustness of such framework. [IOW, I think at WotC rather than spending a huge effort to come up with a micromanagement system wanted by a minority, and them probably ending up with something that the majority of such minority will criticize anyway, they'd better off just saying to such minority that they should create their own system that perfectly suits their preferences]