I'd pick DFs, but ideally haki needs to exist in some form to control logias and hax, as well as explain top tiers who don't use DFs.
Personally I think haki needs to:
a. Be less complicated with more consistent indicators.
b. Not be hereditary
c. Actually sync more with DFs to be unique
For example using haki to buff and evolve your DF power as well as countering others.
Like imagine if Akainu's super hot magma that can burn fire and the ocean was a result of his haki comboing with his fruit
You could even connect haki to awakenings that way - a DF awakens when the user's haki is strong enough.
Another thing would be haki being more specialized, thus giving different hakimen their areas of proficiency. So basically more Vergo types.