There are plenty of preseason angles to attack with out there. There really are.
It takes the right matchups of teams that are working on specific areas of their game. The first step is to go team by team and see what the teams will be working on during the preseason. And what they won't need to work on until it gets closer to opening kickoff.
Understand things like poor defenses in the season before will get worked on during the preseason and depending on which aspect was poor, it could make a difference when they face certain teams looking to improve certain areas of offense.
The same things is true for areas of the game that won't be worked on during the preaseason.
Major cahnges, newplayers, etc. will get more work in the early preseason and as it gets closer to opening day, it becomes more about refining.
Harbaugh and Carroll actually want to win these games when it comes down to it.
Know that a coach isn't likely to try out a new offensive line or schemes while his star QB is under center, especially against a team working on the defensive line performance.
The gameplans unfortunately don't usually last the whole game and shit can fall apart toward the end, spoiling spread bets. But that can happen in the regular season too.
Be careful with the units in preseason, it's less a test of stats and edge and more a test of the ability to process a subjective side. It can also help prepare for the regular season.
I could go on, position by position and what the patterns are for most of how those positions or groups of positions get worked, or not worked, on in the preseason. It also gets a better looking at the coaches as well.
Betting preseason can be rewarding but patience is necessary to get the right matchup and defining value has to be done in a relative market way. How to determine that value is not something I think I've gone i over in the Forum, but I have done quite a bit on preseason analysis here.
Good Luck.