I dont see how cleric would substitute DS sorcerer in its entirety - im not so fmailiar with how multiclass is going to work as it isnt released in EA so if you know more please enlighten me.
In PnP, caster levels stack for the purpose of determining number and level of spell slots, but class level is used to determine number of spells you can know/have prepared. This means that you are likely to have spell slots but not know any of spells of that level to cast with them. For example, lets consider a character who is cleric 1/sorcerer 10. They would have 5th level spell slots, but only have 1st level cleric spells and 4th level sorcerer spells.
What I could infer from what Larian where saying about "fixing" multiclassing, is that if you have spell slots of a certain level you can know/prepare spells of that level. I.e. our cleric 1/sorc 10 can prepare 5th level cleric spells and know 5th level sorcerer spells.
That's not exactly the same as a divine soul - the number of cleric spells you can prepare will be limited to what you would have at level 1 (wisdom modifier + 2 from your domain), would require wisdom to cast, and would be swappable on a long rest, but it does encroach on it's design space. And you would get a deity tag for extra role play options.
With this level of overlap, I think it would be better to look at more distinctive options for future DLC. I would suggest Shadow Sorcerer, since Aberrant Mind is so overpowered compared to what is currently in the game.