You may leave segs "open" (except for the first intercontinental).
So you book say FRA - MEX on LH xxx for October 2nd, and then MEX - MID (Oct 3rd) MID - MEX - LAX (Oct 10th) All these flts are booked and ticketed. You then just leave the rest as open bookings like LAX - BKK - SIN - FRA. There is no fee added if you then ticket the very same routing after six mo's. There will be a fee of 75 $ if you change that route to LAX - NRT - HKG - BKK - SIN - FRA.
There is kind of trouble waiting: if the tix of the first segs are used those open bookings and the attached file key are (kind of) lost. No way to find it. So you need to contact a TA or the issuing airline with your original ticket to create a new booking. To avoid this I suggest to have a fixed booking for the very last segment, which in most cases will be a transfer anyway. Chnaging this would just be a date change without any fee involved.