Boards like that have been released in the past with DDR1 and DDR2 I think it was, and they had pretty crap reviews. So I doubt we'll see that happen here. Likely we'll just see boards that are either DDR3 only or DDR4 only.
DDR3 on skylake is really only an option I'd recommend to someone already coming over with a crap ton of existing DDR3 or just cant get the budget up for DDR4. Keep in mind that DDR4 has a much worse path curve for performance as compared to DDR3. I think the last report I read said in order for DDR4 to bypass the speeds and timings we're seeing with DDR3, you'd have to start with DDR4 3200.