First off, define "soon to be emperor". Is there actually a concrete reason why your husband is about to inherit? If not, see if you can make it happen. If you can't, do your best to keep him safe, unfortunate things tend to happen in these kinds of situations.
Next, do you already have a child with him? Is that child your as well as his primary heir? If either of you have prior children from different marriages, get to the murdering. If you don't have a child yet, seek ways to boost your fertility (WoL focuses!) while you still can.
Now, this is probably a stupid question since you undoubtedly already checked, but is the HRE on primogeniture? Make sure it stays that way; form an alliance now if you can and renew it when your husband inherits, come down hard in support against any faction trying to change the succession law or install a claimant.
Once your husband has inherited the title, and your son is the heir, resist the temptation to kill your husband to put your dynasty on the throne sooner - an adult is way better at keeping hold of the throne than a child.
Finally, do what you're presumably already doing; grow the strength of your realm. You'll need it, because now you've got two countries to defend and still quite a while to go before you take direct control.
