Ill keep this short and simple, Restoring the means of restoring the Achievements that can "No longer be obtained", Such as
for Minor, Moderate, Major, Marvelous, and Master Miner:
-Bringing Mineral deposits on Clockworks Gates back, Which would make Moonstone and Valestone No longer useless. (Unless Three rings has something BIG planned for their future, which I Sincerely doubt!)
And for "Walk it off":
-Bringing back picking up a downed party member by sharing health. Which would also make Playing in a party more attractive and restore the actual feeling of "Teamwork" to the game.
In my opinion, the removal of these functions was both completely unnecessary and highly game breaking, wouldn't you agree?
So your solution to legacy achievements is bringing back legacy systems?
The only reason these achievements are legacy now is because the game has changed.
Mineral deposits were removed because people were depositing valestone and moonstone in massive numbers. This lead to what we like to call the arcade's "ice age". Every floor in every gate was freeze or poison themed, else a boss stratum. This was not healthy for the arcade and was thus removed.
While I want a use for valestone and moonstone as much as the next person, I believe the way to go about it is not gate construction. If moonstone and valestone had uses in battlesprite food crafting that would be reasonable, but unless they have a more "useful" use outside of gate construction then we should not bring gate construction back.
Health revives were a wonderful asset, however when battlesprites were dropped the game shifted more towards a "carry yourself, build a team" instead of "rely on your team". This change was very helpful as it means if you can't handle yourself in difficult areas it is up to you to improve, not your team's job to keep reviving you, having you go down, and then have everyone be equally weak, leading to a team wipe instead of just a player wipe.
While I don't think health revives should make a comeback, I think sparks of life are too solo-friendly, and not team friendly enough. If for every spark of life you got you also found a handful of party revives, which I have come to give the nickname of "Life Essence", that would be great. Life essences would act like health revives in that you would have to attack while above a downed teammate to revive them, however it would act like an emergency revive, only filling up red health (alternatively they could only revive you at 50% health). These could only be used on teammates, and would not obsolete Sparks of Life. You now have bound revives that you can only use on teammates, so you're more likely to party up and use them. It's a win-win.
Bottom line is that legacy achievements are legacy because the game changed. If you're a steam completionist then yes, the legacy achievements should be removed from Steam, but be retained in-game.