With the heavy merge coming northbound at Exit 13 and the short distance to Exit 14 (<1 mile), I can't see why not a 3rd lane could be added in both directions between Exits 13 & 14. It doesn't necessarily have to be a thru lane, especially if the cloverleaf is modified with a c-d road approach. The 3rd lane can be an "operational" lane, basically combining the Exit 13 acceleration and the Exit 14 decelleration (and vice versa southbound). You wouldn't have to replace any bridges, either (except the US 2 over I-89 bridge, but that would be replaced if Exit 14 is modified I'm sure). North of Exit 14, you have one overpass and maybe one underpass to worry about. The Winooski River Bridge is already 3 lanes each way, so really you've got about a mile of widening in each direction north of Exit 14, and the 1/2-3/4 mile between Exits 13-14.
Its interesting to note that Vermont's interstates have retained their same number of exits (and basic ramp configurations) since day 1, and that, outside of some lengthening of acceleration/ decelleration lanes, no widenings have taken place on the interstates. I can't think of many other US states to have these same claims, and I am sure there are those hardcore ones who will say "leave I-89 alone" (and they're probably the ones that got Exit 10-A aborted - I'm looking at you, Bolton). After having spent some 15 years in VT, leaving in 2018, I can tell you there is indeed the need for at least a mile or two of widening of I-89 in the Burlington area.