That formation is not my preferred formation at all (I did not mention anything about what I would like to see) but more the formation that I expect Valverde to use as I cannot see him dropping Suárez although I could be very wrong.
I like your last formation if not for the fact that Griezmann has never really played as a "right winger" from what I am aware of. Messi will continue to operate primarily in that area of the field while continuing to spend more than half of his time in the center or wherever he wants to (it seems) with Valverde around.
Buying Neymar rather than Griezmann (if buying them in the first place) would probably be the best option tactically as Neymar would be guaranteed the "left wing" while Messi could room the center (false 9) and Dembélé the right wing where I believe he is at his best. Dembélé teaming up with Arthur and Semedo would also give the team the needed spark to our entire right flank.
Anyway I could very well see Valverde use a 4-3-1-2 formation with Suárez and Griezmann upfront and Messi as a traditional number 10 with license to room freely around. Not sure where Coutinho would fit in such a formation and who would provide width other than Alba. We would have the same problem of a mostly entirely static "right flank".
I agree with all your points. Having Dembélé and Semedo together usually works well despite the lack of consistency they've both had. I think they'd develop quite the partnership with some playing time.
It's nice to play around with formations when there's a lot of potential like now. But at the same time you know Valverde won't do anything drastic. As you're saying, Suárez, Messi and Griezmann will be undisputed starters.
And the midfield will most probably consist of 2 veterans.
It's kind of a crucial moment in the club now with a lot of big money signings moving. Dembélé/Coutinho: who will leave? Neymar/Griezmann: Who'll be signed?
There's a lot of pressure on Abidal and the likes and they should be fully responsible for these choices. It's pretty certain that Valverde will leave soon, plus I wouldn't trust him with decisions like this.