Talk to teachers from the grade below, think about what hasn't worked well in your class the last few years, think about how your school district might differ from that of the lesson plan you're modifying.
For example, maybe you need to modify some take home science experiments because your kids tend not to have easy access to the right materials. Or, maybe the elementary school math program is more advanced than average, so as a middle school teacher you need to modify any standard lesson plan so you're not repeating material the kids already know. Or, maybe as a Spanish teacher you need to figure out how to effectively deal with a class where usually half the kids are children of immigrants and already close to fluent. Or, maybe as a history teacher you have an assignment the kids used to love where they make a Facebook profile of a historical figure, but now most kids aren't using Facebook anymore and the assignment isn't really engaging them. Or, maybe your town has some historical sites that you'd like to make field trips to, so you want to add some lessons to give some specific context around them. Or, maybe as a P.E. teacher the gym is being renovated and you're gonna have to work around that. Or, maybe a local private school just closed and you're expecting to get a bunch of its students in your class next year. Or, maybe a lot of your kids can't afford TI-84s but the school FINALLY got funding to purchase enough for the whole class. etc. etc. etc.