D&D 5E Could we do with one more skill?

[nomotog]

The way I picture it. If you have a player who wants to rig up a pulley, it's a simple int check. If you want a class, race or something to be really good at engineering I say you can add it as a new proficiency. (In my mind, I blur the line between proficiency types.)

 

[juggerulez]

Those uses of ability scores that do not reflect on an existing skill are simple ability checks.
If you think his knowledge should fall to a specific skill, you can create one ex novo by "averaging" two existing skills or create it from scratch. The latter option, though, will decrease your player's character skill pool from a side, and make him an absolute expert on that field to another side... I think it's not advisable because the implications could create more issues than those the new skill is meant to fix