Ndombele fdj Arthur would be a pretty solid midfield. Might lack a bit of creativity but I think Messi and Dembele could make up for it.
You have to think about other options.
We were quite blessed that Messi was not injured at all in the last few years.
He will be 33 at the end of a season:
1. what if Messi retires, imagine having a Frenkie-Arthur-Ndombele midfield then (without Messi around). We would need to sell/rebuild/reinvent things again after that
2. imagine if Messi gets injured for 3 Months in this season. Our only creative attacking player would be Dembele
3. imagine if Dembele remains injured or out of form (due to injuries or other reasons). Then we would have only Messi and even worse version of Messipendencia since in the last few years we had creative midfielders in Xavi and Iniesta, and now we would have nobody or maybe Frenkie (we need to see how he will fit)
So, on paper, that midfield doesn't look that bad as long as Messi and Dembele are here and fit.
But again, Messi is old and might get injured.
Dembele had 5 long injuries in 2 years here.
Not to mention that we already have 4 players who are hybrids of a pivot and CM: Busi, Frenkie, Arthur and even Rakitic.
Too many similar players and now even the 5th guy, Ndombele, who would cost 70-80M.
If we have enough money, fine.
If not, it would probably be wiser to invest in other areas or in different type of midfielders.