Hiya!
Well you should be happy
In terms of overall content, feats were far and away the most requested new element. Over 70 percent of you want more feats for your game. Feats also had the least opposition to their expansion. Because we haven’t released any products focused on new feats since the launch of the game, we know there’s definitely some pent-up demand for them. But feats are perhaps the thorniest element to design for D&D, as they tend to span multiple abilities and can trigger weird interactions that the game’s core design can’t always account for. As a result, we’ll be taking things slowly as we explore options and starting points for new feat development.
I'm
not one of the Feat wanter's. I have absolutely no interest in them...but I realize others do. Apparently 70% of those surveyed do... So, the thing I hope that WotC does when they come out with a 'crunch-book' (whatever they call it), they don't make one...they make multiple, smaller ones. Ones that are focused on some area. So I don't want an "Unearthed Arcana" for 5e, I'd be happy with a "Book of Feats" for 5e. Why? Because I can than just not buy that one. They can come out with five or six different "Handbooks" (ala 2e's Handbook series). That way I can not buy those (or the ones I don't want). They can come out with an "Arcane Spells" and "Divine Spells". That way I can look them over and decide if it's worth it to me. So, as long as they don't say "Here's a 256pg book for you...", of which only 30 pages are going to be useful for me...go for it. I guess.

It's going to make having conversations with 70% of the 5e crowd more difficult for those of us who don't use Feats, Errata, MC, etc...but...whatever.
Now, that said...all that they really need to do to "fix" unforeseen uber-combo's they hint at is to keep writing in the tone of how they wrote most of 5e's rules. That being "semi-vague, regular language, with DM caveats sprinkled in liberally". If they can say
"This allows the character a better chance of detecting things that are hidden, obscured, or otherwise not normally noticed" over
"This allows the character Advantage on all Perception checks regarding opposed Stealth rolls"...that would be a good thing. In the first instance, it leaves the exact details up to the DM and Players to decide. In the second, it's a munchkin-power-gamers wet dream as it gives them something they can point to and say "
See! YOU are WRONG...stupid DM! Ha! My combo wins!".
So..when they do write out the HUNDREDS of Feats they will inundate the 5e community with, at least do everyone a favour and err on the side of "less, not more".
As for the rest...classes, races, etc? *shrug* I'd
much rather have WotC make crunch books focused on helping a DM run a cool, believable and exciting campaign over giving Players more options that a DM has to deal with. Give me a Wilderness Survival Guide, a Dungeoneers Survival Guide, a Cityscape Survival Guide, a Castles & Fortifications Guide, a Seaborne Adventures Guide, a Guide to the Inner Planes, a Guide to the Lower Planes, a Guide to the Higher Planes, etc...etc...etc... Inside of those books...toss in a couple pages of Feats specific to those 'things'. Add in a few pages of new spells, arch-types, equipment. That's fine...as long as the majority of the book is there to help the DM run cooler games.
My 2¢ anyway.
^_^
Paul L. Ming