Yeah 747 classic is what I would like to see first, CS 727 with proper third party fixes is good enough already, CS shouldn't waste time on improving it more when we are completely missing many great classic airliners.

This would be my priority list:

1. 747 Classic, 747-100 and 200 as base pack, then cargo version, -300, 747SP and maybe some military versions too.

2. DC-8, maybe first variants as base back and later as expansions, + of course cargo version.

3. DC-10, there is really not that big differences in outside of different models, so maybe all pax versions as base pack and then cargo and tanker versions as expansions.

4. L1011, maybe those bigger versions as base pack and then -500 + cargo versions as expansion.

5. Il-76, only ones are from FS9 and really don't work well at all with FSX, myself I had to make custom 2D panel with fs panel studio to be able to enjoy one.