Hey always gotta kick the hornets' nest, every now and then ...
Given the simple 4e -> 5e official conversion
that 4e level 3 = 5e level 2, multiples on up,
I think that would make every 4e Heroic feat
open to 5e e6. Maybe some of the Skill Feats
that also take the level 10 Daily, don't apply.
A few more "ought" to be limited to the power slot.
But 4e won't imbalance 3e/5e stuff all that much.
E8 otoh would open up 4e level 12 abilities
and there'd need to be a finer toothed comb
to make sure unintended interactions, don't
spool off into crazy town.
Or maybe they should spool off into cra-cra
The 4e add-a-skill feat with a +5, would still be
tasty if allowed with 3e UMD. Could even function
as a plot-line effect. DM hands out a magic item
that none of the PC's classes can use. Instead
of 1 dude being grumpy about multiclassing,
1 dude gets to be less grumpy, uses the item
and can use more off-class items per DM whims.