Both may very well be likely. Right now City just have to secure Top 4. I can't imagine that changing the manager improves that, so the summer will be very interesting. Manuel probably needs to show a willingness to adapt, but the current players executing the current system will likely be far more than enough to get the results needed to stay in the CL and keep the financials solid. That has to be goal #1 at this point.
I think a lot of the player transaction stuff depends on who is going to manage the team and what they're going to do with Yaya. 4-3-3 makes boatloads of sense, except it puts somewhere around a hundred million pounds of striker on your bench. Aguero is an incredible player and City are lucky to have him. He's worth building the team around and I live in fear that I'm going to wake up one day and Aguero has decided he wants to move to Madrid. But when combined with Silva, it becomes a very tricky problem to solve. Aguero can't play the role of target man striker, he just can't compete physically and it all but rules out playing any formation where he is a lone striker. Whenever he's been alone up top he's been isolated and City have no pressure release on a long pass unless it is to feet or over the top. You can of course imagine Aguero in a 4-4-1-1, but Silva is the natural #10 in that formation, and now you've got two midgets as the release valve, which also doesn't work in practice (and it's what City went to against liverpool, not 4-5-1, strictly speaking). So you're almost stuck with 2 strikers, and one probably has to be a big guy to help the midfield and defense and accomplish some hold up play. Then if you incorporate Silva, who is absolutely worth incorporating, you're starting to have the makings of a 4-4-2, or, as you pointed out, a 4-3-3. I think the formation that probably makes the most sense on current personnel would be a 4-3-3 with Aguero left, sort of like Messi or Neymar, cutting inside and having a free role with a large striker like Bony or Dzeko as the tip of the spear with Silva free on the right. This would essentially free all 3 of them from defensive responsibility, which is essentially how they play today anyway, but would not require the front 2 to press the ball, which they simply haven't ever done.
But if City were to move to a 4-3-3 in this football manager SIM, they probably need to sit Toure back in his Barca role, hoping he can tackle well enough to play at the base, because I don't think he can shuttle as an outside mid in a 4-3-3 consistently enough to provide the defensive solidarity you'd want this change to produce. Fernandinho can easily play this role for a year or two, and Milner probably could, more or less, but realistically City are lacking that Fernandinho clone to make this work. Seeing City linked with Koke and Pogba makes this make sense, as either of those players can likely play that role, but realistically they'd need two very, very good midfielders to have the squad depth for the formation. In any case, a 4-3-3 would probably give City some additional defensive solidarity, although they'd have still been outnumbered against Liverpool in the midfield. But right now the problem is that it's really a 4-2-2-2 and even the base 2 are getting forward in attack (Fernandinho and Yaya) while Silva, Nasri, Dzeko, Aguero do absolutely no pressing of the ball to win it back. It's just suicidal attack after suicidal attack, and the defenders are being left far too exposed. If City really wanted to play 4-4-x, then Silva and Nasri can't both occupy part of that 4, it just doesn't provide enough cover.
I think I'd add that the reason why this system worked so well before it didn't is largely the evolution of tactics against City. By the end of Mancini's run, any team that wanted a result could capably set out to park a bus and probably at least draw the match. City consistently faced 11 men behind the ball and really struggled to unlock this until Pellegrini showed up and committed nearly everyone to the attack save the two CBs. As a result, the goals flowed against these packed defenses. But the teams have learned to adapt and have certainly learned that you're not going to survive 90 minutes against this version of City by simply trying to absorb their attacking intent. The natural counter to all of this has been to take advantage of the space City leaves through the attack commitment, especially considering the poor commitment by City's players to winning the ball back as quickly as possible. It's for this reason, more than any, that I think a managerial change and a large squad change may prove necessary. If City were to entice Pep or Simeone or someone to City, they'd inherit a squad that simply isn't equipped to play football the way it is played at the highest levels right now. Maybe Nasri and Silva and the like can play this way, but Pellegrini was known for this style of football before coming to City and just can't seem to get the players to press the opposition to go along with the high line. I don't know that another manager can change that, that's largely about the mix of players and their skillsets.
