They’ve mentioned that an upcoming unannounced product will include rules for customizing ability score increases from “race.” I think it’s far more likely that they will print updated core books that include these optional rules, and possibly some of the player options that get frequently reprinted, such as the spells from the EEPG and XGTE, the underdark variants of gnomes and dwarves, possibly Eladrin, maybe the Tiefling variants, etc. and likely playable Goblinoids and Orcs.
I’m not sure that an optional rule system for playing classes will cut it. From my perspective, it simply bakes in the race issue and it nods and winks at those who want to persist with questionable D&D origins. (Talk about digging a hole for oneself!)
If WotC claims to be inclusive and diverse, they can only do so by drawing a line under 5e and introducing a new edition. Sure, they might claim “5.5e”, but a shift to a class based system is a big difference for 5e. For instance, a class system means ability bonuses, saving throws, proficiencies are class based, not racial. There’s no being proficient with an Dwarven axe merely because you are a dwarf or a Longsword or fey ancestry allowing acces to bonus spells because you’re an elf.
Perhaps, counterintuitively, a system of “alignment” is still needed. It might be called motivation, ethos, or some such. I could see this working if it was given the sort of detail that alignment was given in 3.xe and inspiration in 5e. This motivation / ethos / attitude would be the means to guide and define classes, such as ranger or paladin from fighter, a warlock from wizard, etc.
Differences between each genus / family / ancestry could be restricted to DNA - darkvision, low light vision, movement speed or special features (dragon breath).
There might also be preferred classes. It might also involve skill modifiers and feats, but this would need to be handled REALLY carefully. For instance, like in 3.5e, I can see that humans life spans might allow them access to an additional feat. But, suggesting a rogue nimble fingers feat for a Halfling or an innate rage for a Half-Orc is irresponsible.