Not sure that ideal #6 exists today, I sure don't know about anyone.
I think they do. We just have to bid for the right ones. Weigl is the most obvious choice with his composure and tactical discipline and passing range. A bit more unrealistic, but Verratti is someone who absolutely has the skillset to be an elite holding midfielder, and has already played that role with Rabiot + Matuidi just ahead of him. Paredes from Roma, Trigueros from Villarreal, Llorente - who's on loan from Madrid, young Locatelli at Milan (might be a bit too early for him, though), potentially Neves. Then there are players who can be converted (Koke, maybe Goretzka, and Dahoud - who has the potential to transition from attacking midfield, ditto Óliver). United's scouting department just needs to take a deeper look at the options with that specific profile in mind if the obvious ones can't be signed.
Tolisso, Herrera and Pogba in a midfield three would certainly give us a lot of agression and mobility though, Tolisso got that buzzard-like quality that Herrera also has.
That's the thing, though - ideally, you need to have a broad spectrum of qualities between the trio. We have enough mobility with Pogba + Ander (especially in offense). So that range of skillset is covered. What you need now is someone who can bring composure and creativity to our passing, 'conduct' things, and become the primary brain to Ander's buzzard-like quality and aggressiveness. We often tend to lose our coherence when Carrick is not on the field, and that needs to be addressed, above all else. Pure defensive quality is a secondary concern, because good holding midfielders can stem the flow of opposition attacks with their positioning, as opposed to physicality or aggression.
If you look at the elite European clubs, they all have atleast 1 excellent passer of the ball (apart from Atlético where the wide midfielders tuck in under Simeone to compensate for it):
Madrid: Modrić + Kroos
Barcelona: Iniesta + Busquets
Bayern: Thiago + Alonso (when he starts)
Juventus: Pjanić
And so forth. We currently have zero of that sort when Carrick is out (Ander a good passer, but not someone who can impose his will on the game in that department).
Even at Madrid and Chelsea, Mourinho had Alonso and Cesc (more advanced, but similar in terms of function) who got 100 touches per match and helped open the game up with their passing ability. Having someone with that quality is priceless: no amount of aggression or mobility will make up for it, and United fans should be aware of that given our history with the likes of Scholes as the deep lying playmaker when he aged, and the understated effectiveness of Carrick - who subsequently took on the mantle of our passer/playmaker in chief: