I posted about Engineer earlier, but I thought I would bring it up again.
Goblin Engineer feels absolutely insane. You don't get to do the broken things that Welder is capable of but Goblin Welder always had the issue of consistency. Welder needed set it and required Dacks, Thirsts, Bazaars, multiple copies of similar spells. Engineer doesn't require any of that. Play it with singletons. Go get a Crucible when it's good, a Bridge when it's good, or Time Vault when it's good. Go get Sculpting Sphere and Tangle Wire. Or just go get Trinisphere. And you don't have to worry about @nedleeds favorite card, Misstep.
I feel Engineer is actually an upgrade on Welder, which is odd to say as I can definitely appreciate the historical implications @Smmenen alludes to of Welders past dominance. It's just Engineer feels a modernized version of the card that corrects the flaws and opens up new potential. I'm pretty excited to see what it can do in Vintage and Legacy. It's already making waves in Modern with Thopter/Sword + Urza. Give it a try.