Ok, so this is some wild speculation on my part, but this is what I'm expecting based on the seminars, columns and rumors.
We already know the checks default to Abilities, right?
So three 5E characters, a Basic, Core and Advanced walk up to a narrow ledge across a gaping chasm. They want to cross it.
1. Regardless of their complexity level, the first thing they do is say: "I cross the ledge."
The DM makes a judgement call, informed by context, common sense and ultimately, rules. It's a DC 15 ledge and the character has 17 Dexterity? Ok, done, ledge crossed, play continues.
2. But maybe the outcome is uncertain, or the character's Dex is lower than the DC. All three are asked to roll a d20 and add the appropriate modifiers.
3. The Basic character adds her Dex modifier. She can get an Advantage (like Combat Advantage, but awarded by context in any circumstance), if the fiction dictates it. Plus her Race/Class/Theme can give a generic bonus. Frex, elves are nimble and graceful and merit a bonus. The DM can handle that out of his head. No fussing about for the player, she only needs to know her Dex score.
4. The Core character adds her Dex modifier plus her Athletics skill bonus if she has it trained, plus maybe a feat bonus and stuff. There's a little customisation, but not a ton of it. This is basically the same as 4E, but there's no 1/2 level bonus because it's not necessary as the math doesn't escalate as hard. The final bonus is on par with the Basic guy.
5. The Advanced character add her Dex modifier plus Athetics skill ranks which are gained at some rate, plus feat bonuses plus maybe some skill synergy bonuses, plus racial modifiers etc. Put together, her bonus is most likely higher than the previous two characters, but not significantly so and she's probably slightly lagging behind them in other areas because of customisation/specialisation.
I think that's entirely workable and achievable. The design team just needs to make sure the bonuses are more or less on equal footing in a final analysis. I doubt we'll see such pure mathematical balance as in 4E (and they've confirmed as much) across the three types, but that's the price to be paid.