I have been told that we cannot expect to get custom classes, but I'm confused as to why.  For example, if I want to have available the sub-classes listed in Xanathar's Guide to Everything Else, which is not a WotC publication, I would have to Homebrew the info for Dragoon.

Fantasy Grounds, while being different (a VTT) allows me to build these things, but I'd rather players track and level up within dndbeyond because frankly, it is a lot easier and more accurate to level up and track things in it.

I'm hoping not to get confirmation about classes, but to understand the official reason why  we cannot expect to be able to homebrew classes. 

Because full classes are complicated and can introduce an arbitrary number of subsystems and changes to the rules. Look at the Warlock class: it has non-standard spell slots, and besides picking a subclass it also asks you to pick a Pact Boon and Invocations. The invocations can have subclasses, pact boons, or warlock level as prerequisites, and new books can add new invocations like XGtE did. Coming up with a user-facing system that can do anything you might think of, with all the rules and restrictions and recalculations, is really hard.

Subclasses rarely change their class's rules, just add new limited-use abilities or resources (e.g. Superiority Dice.)

So all of that is handled in Fantasy Grounds without any problem. As a developer I understand the complexities of coding something like this.  However I'm not asking for a system that allows me to invent my own rules.  I'm asking for a system that lets me operate within existing rules.

Example: Dragoon from Xanathar's Guide to Everything Else...

At certain levels you grant certain Features... this is common across ALL classes.

Certain Features grant advantage on certain rolls, in addition you could write any flavor text you'd like without attempting to affect the core mechanics...just kinda to document it.

lvl 3: Versatile Combat ... you gain weapon proficiencies, extra damage while mounted, and a little extra speed. This is dependent on the type of armor you're wearing. if you're wearing another type you get extra bonus to hit. and a further speed increase.

So ... what would that look like in a "simple" system?

[Add proficiencies]

Conditional:

When [wearing medium armor]:

• apply [extra damage]
• apply [extra speed]

When [wearing light armor]

• apply [even more damage]
• apply [even more speed]

In that circumstance everything listed is already a modification they have code for with perhaps the exception of detecting the armor type worn.  Either way, this isn't insurmountable.  Most class features are actually not even something to be calculated within dndbeyond, they are in-game calculations... like pushing a creature to the side when charged.  The only thing that would be handy dandy D&D beyond would be a DC for that... so apply a DC calculation if applicable...

Flanking Maneuvers would have no dndbeyond calculations necessary at all:

At 10th level, your battlefield awareness allows you to sense a retreat and cut it off as it happens. When you hit a creature with an opportunity attack, you can move up to half your speed immediately after the attack as part of the same reaction. This movement doesn’t provoke opportunity attacks.

Starting at 18th level, if you make a melee attack against a creature, that creature can’t make an opportunity attack against you for the rest of your turn.

From what the devs have shown, the character sheet revamp will handle ad-hoc customization if you're just looking to give yourself extra feats, proficiencies, bonuses, custom AC calculations, etc. That should be enough to at least get the statistics right.

I doubt the class features field will be editable but worst case scenario you can type them up as homebrew feats.

Quote from InquisitiveCoder >>

From what the devs have shown, the character sheet revamp will handle ad-hoc customization if you're just looking to give yourself extra feats, proficiencies, bonuses, custom AC calculations, etc. That should be enough to at least get the statistics right.

Thank you @Stormknight!