Possible design depends on how much redundancy you want to achieve.
You mention using another router for redundancy. Okay, that's fine, but what about redundancy to/from the dual routers, both on inside and outside these routers. For example, what, if any, redundancy options are available to/from the destination of the IPSec tunnel?
Do you want a cold spare redundancy, warm standby redundancy or active-active redundancy? With the latter, if one router fails, is loss of capacity a possible issue?
You need to clarify how much redundancy you need/desire beyond just having another router.
At the simplest, another router might be set in parallel to your current router and would backstop just your current router.
In more extreme redundancy, you try to have a parallel path that one failure will not break connectivity on the other path. (Personally, in my experience, dealing with redundant WAN fiber sharing the same trench or POP was the hardest to handle.)
