Sets need to be somehow improved/redone for being more useful than character fully equiped with low-mid tier crafts. It'll be possible to do in Sigma with ability to make any item being locked to one character so noone exxcept certain build would be able to use set parts. And i think they need some unique stats for build they done for.
Some examples: full bowzon set may add pierce to dragonlore skill that scaling with baselevel (instead of -all resist currently) or smth like "wyrmshot fires missiles 25-50% faster".
Melee necro set could have bonesplitter transformable into spirit edge and improve deathlord morph by adding there some skill IAS scaling with hard points (to compensate bonesplitter slowpoke attack speed and make it easier to reach last breakpoint) and adding extra 1% more base block chance at some base skill lvl (10/15/20) to make it more viable to put hard points in it while making choise between scythe and dagger less obvious (me personally almost never using scythe anymore after attack speed nerf since daggers can provide nearly same damage while being much faster)..
Light sorc set can simply double avoid from warp armor (currently you not able to get more than around 20% AFAIK... if not uber-end-game with Khanduran, upgraded Laz, all charms and so on) and/or double tempest bolts.
Cold set (currently cold sorc is least useful of all existing) is already good at defence stats but still ice bolt nova is not good enough for being good further than a5 terror.. it could alter/improve abyss somehow. or even ice elementals since now they're more offensive than defensive.
And so on with every single set. So they will become smth more unique for every certain build and will get alot more reasons to use. That's what i think about them for loooong time