When watching tape of a team you play this year that lost a lot of players from the previous year what do you focus on? The overall scheme? Or the players that are returning?
Scheme. Unless they have someone who is notably dangerous, I usually just focus on defending the scheme.
However, if they lost a lot of guys, you may also find that the scheme they showed last year is going to change a lot this year. If they had a great qb and some pass catching threats who are all gone now, obviously you don't need to be as concerned about the passing game. Basically, I'm going to get all of the scheme I can chart, evaluate who isn't back, and try to determine what they'll run based on the scheme I've seen.
Most teams aren't going to fundamentally change their offense year to year. The balance and mixture might change, but the schematic stuff carries over mostly intact (barring a staff change, anyway).