In fact, thinking about the Dier backup/second B2B midfielder requirement, we might be able to get away with combining both into one if we can find a player who's both adept at counter-pressing and retaining the ball under pressure while taking it forward (i.e, Dembele), and capable of dropping between the two CBs, spreading play and making interceptions (a la Dier). A couple of those do spring to mind: Saul Niguez, for example, who plays for Atletico, is that type of player. Relentless, combative central midfielder who makes a lot of interceptions/tackles, and is defensively adept (by virtue of his time spent playing at centre-back for Rayo Vallecano a couple of seasons ago) but also great on the ball with a good pass on him. Only 21, too, and he's been vital to Simeone's high-pressure system this season.
Problem is, he apparently wants to stay at Atletico until he keels over, and they're rushing to offer him a new contract (since his current one has a release clause of only 24 million euros). Another option could be Riechedly Bazoer, the young Ajax midfielder: he also started out as a centre-back, so he also has defensive prowess to accompany his B2B skills (and he has everything in his locker: pace, power, strength, aerial ability, dribbling, passing, the lot). The problem with him is that firstly, he's already apparently got a clear career plan: he wants to be at Barca in three or so years, and has been coordinating that plan with both Ajax and Barca, meaning the prospect of him leaving Ajax is slim, and him staying here past a couple of years is equally improbable even if he did arrive. Plus, apparently he's confident in his own abilities to the point of ignoring tactical instructions and arguing with his coaches if he feels they're wrong, and that might not go down too well with Poch.
A third option could be Granit Xhaka, at Borussia Monchengladbach: again, he's combative (throws himself into tackles, is strong, is taller than Dier and wins his fair share of aerial challenges), great on the ball (especially when it comes to long passes) and routinely alternates between pressing high up the field and dropping in between the two CBs during moves. He's also a leader (captain of his side at the age of 23), and his work ethic would fit well with our lads. Problem with him is, apparently Arsenal have already talked to his dad, and (if Sky Sports Germany are to be believed) are lining up a 30 million-quid bid for his services. If he's the one we want, we'll need to move quickly, since this isn't the only report of interest in the lad (City apparently also want him, as do Bayern). He also isn't the fastest player, and he's a bit of a red-card magnet: six in just two seasons.
Overall, the fact that these players who combine defensive and offensive abilities are so rare is what makes them so valued: but if we want to solve two problems with one solution, this is the way we should go, imo. That way, we get back-up for Dier who can also play alongside him if necessary (and, alternately, play alongside Dembele if Dier's out) and strengthens our side, while also leaving enough midfield slots open for youngsters from the academy to be put into the squad and given the feeling that there is a firm pathway from academy to the first eleven for them if they're good enough.