An important aspect of Alonso's game that will need replacing is his presence from goal kicks; he was a very effective outlet aerially. I'd expect Lukaku to fill this role, however, as he's far far better in the air than Costa is. Given the strong indications that Conte's top priority is to bring in Sandro and VvD, I'd expect to see a first choice wing back pairing of Azpilicueta and Sandro next season, which I think makes a lot of sense. Azpi is better going forward from the right side than he's given credit for, he puts in a decent cross with his right foot if not with his left and his shooting isn't any worse than Moses' for my money. This also fixes the main vulnerability of last season's 3-4-3, namely Alonso getting roasted for pace and crosses coming into the Moses/Azpilicueta zone at the far post.
I'd also like to see Chelsea go after someone like Bernardeschi, especially as Hazard is recovering. Plus after he's back it would be nice to have a left footed wide player as a different sort of option.
Thus, my ideal version team would look something like this:
-----------------Courtois-------------------
-----Van Dijk-----Luiz-----Christensen----
Azpilicueta(C)-Kante--Bakayoko---Sandro
------Pedro------Lukaku-----Hazard------
Bench: Caballero, Cahill, Alonso, Willian, Bernardeschi, Fabregas, Batshuayi
I personally think Christensen is already better than Cahill, though this is probably not going to happen from day one. All these moves seem pretty plausible, and would go a long way towards avoiding another post-title calamity.