Similar to how air units can return to air staging facilities to be repaired, I believe land units should have a button that marches them home to the nearest engineering station (maybe with a shorter range than the air staging facilities) to be repaired.
Also, engineering stations should automatically repair units near them. When I was first learning the game that's what I thought they did, sorta like a stationary mechanic or field hospital. However they don't do that, and I think it'd be pretty neat if they did.
There's been some great discussion on the viability of increasing repair speed / decreasing repair costs so I'm not going to comment on that. Besides, I'm only rank ~300 or so on ladder so my opinions aren't worth much.
Right now repairing is an underutilized mechanic. Whether that's good or bad is beyond me, but the fact is that people don't use it. I think this suggestion would add an automatic method of using it, which helps prevent it from being the APM sink that it is now.
In terms of how this'd affect the game, I believe it would encourage players to keep their units alive rather than throwing them to their doom. This encourages players to be less wasteful, and it gives the engineering station another role besides "passive stationary build power" which could already be accomplished by spamming engineers.
It'd also add additional flavor to the space behind the frontline / firebases, which could do with some additional utility. Right now firebases are mainly used to control space, but with an engineering station repairing units just behind them they could act as a rallying banner for previous missions to regroup at before again pushing into enemy territory.
The way I picture it working is the player selects a group of units, some damaged some not, hits the "return to engineering station" button, then queues up an additional movement order for afterwards. Essentially, "repair yourself if you need it then meet up here" - this would allow the player to send some damaged units home partway through a mission, while keeping the undamaged units on task. Sorta like how if you select all your air units and then make an order that only some of them can follow (like, shoot down that spy plane) only the interceptors will move to engage while everyone else will do nothing / do any other orders you specify (like "shoot down that spy plane and then return to an air staging facility" causing the bombers and spy planes to immediately go repair+refuel)
I believe this adds an interesting decision for the player to make - do I send my units home to repair, or do I push further and risk losing them?
More meaningful decisions == more gameplay == more fun.
This is my first suggestion so let me know what you think.