This.
I like Sig, but as always people are missing the point in regards to how the manager wants to play.
Looking at S'ton, Tadic was often utilised as the de facto 'playmaker' over the past few years, but he was hardly played central. He's a winger who creates, but he had Mane, Long, Pelle etc around him pressing.
Koeman then had three 'runners' in midfield, but he didn't have a playmaker like Barkley in the squad.
Now, he does, but in order to fulfil that role, Ross needs to be played deeper. He needs space. Now, we also have Rom, who needs support around him.
So, we need two 'wide forwards' to play either side of Rom and do the pressing/neat link up play, and then we can have the three in midfield who provide the energy, but among those three we'd have Ross to pull the strings and if he has players like Gueye, McCarthy/Besic (if still here/fit) and hopefully Schneiderlin (Davies long term) around him we've got the perfect package to also mix it up in game.
That may look like a 4-3-3, but it's also easily switched to a 4-4-2.
It's a huge shame re. Bolasie as he gave that bit of width but also that pace and power up top. But injuries happen and he isn't a natural forward, so it doesn't change the need for that.
Options would always be great, and if Koeman wants to go 4-2-3-1 we need a proper 'no.10'. It's what was missing from Martinez's side for two years once Pienaar got crocked, but we look to be heading in a different direction now, albeit slowly.
That being said, I'd happily take Sig if he was an option.