Personally, I will be using custom backgrounds with pride and delight. Because here's how my ideal Background mechanic works conceptually:
Every story has a beginning. Your character's background represents where you came from, and what your life was like before you became an adventurer. Like every living being, you were shaped to an extent by your environment. That includes, but is not limited to, your previous occupation(s), your social class, the area you grew up in along with its geography and climate, your kin and kith, your upbringing, your culture, all the places you've traveled and all the relationships you've formed.
All this contributed to what you are now, and your Background is an amalgamation of everything you gained during your formative years.
Benefits: pick 2 or 3 stat boosts, an origin feat, two skills, and a tool, take 50 gold, off you go.
From there, every table and DM can choose one of two approaches.
1) it's the DM's job to rein in the background's benefits, making sure they fit the flavour and aren't too convenient for the character's build
2) it's allowed and indeed expected to pick benefits that are most convenient to your build
See, with so many factors going into the flavour (and rightly so, all these things do shape us, you are not just your job!), surely something will be able to justify the player's choices, unless they make something silly on purpose. So I'll take the second approach. But the first is also very doable, if that floats your boat, and I sincerely think it's a much better mechanic than having 16 choices and waiting for WotC's official blessing (custom backgrounds in the DMG, and additional ones in splatbooks down the line) to add a 17th.