Coming back to this because it's something I have thought about a fair bit lately.
For me, we have to be braver and stop taking the easy option of attacking down the flanks. There's a reason teams leave space there....because it's not a threat!
The intention should be to get the wide forwards closer to Ronaldo and then play off him every time. Quick passes into Ronaldo, immediately bounce the ball off to one of the three attackers and launch an attack from there. This has the added advantage of ultimately making CBs more worried about Ronaldo coming short than in-behind, so pulls them out of their block, away from goal and leaves the ball over the top on and space for the wide forwards to attack.
Ronaldo isn't the ideal forward for this, but it's what we have right now, although I'd like to see Bruno Fernandes given a run of games in this role, kind of as a false 9, which then also lets us get Matic and Pogba on the pitch together - giving us more ability to play through the lines.
If Sancho, Elanga or Rashford DO receive the ball out wide, they should drive inwards, towards the centre, looking for wall-passes or little balls in-behind defenders for the CF to run onto. Elanga did this really well at the weekend and it should be the default option. Forget these underlapping runs and looking to get the full backs in, we're not geared up for it. Sure, they should provide support and the odd overlapping run when it's on, but it shouldn't be the main focus of our attacks.