I think it's partially the residual sweetness and strong maltiness that is present in most barleywines. Even in an American Barleywine (vs. English) there should be some sweetness and a definite balance towards the malt. BJCP
says "rich and strongly [or intensely] malty." Its not an Imperial IPA because of this, though the extremes of the two styles can be similar. IIPAs should finish drier.
Just adding more alcohol to a beer style won't make it a barleywine, just an "imperial" whatever style...
Use some Crystal 40-60 malts to accentuate the sweet maltiness, but in general you get most of the color/sweetness from a longer boil and a higher mash temp. Lots of wort, boiled down for a long time. My last barleywine boiled for nearly 2.5 hours (I also way
overshot my OG due to calculation errors in my recipe).