Do I need to increase my HVAC trunk line size?
I'd go with option 4 to begin with. I'd recommend it for a number of reasons:


It's a separate unit, so will act as a separate zone in your house, meaning you don't need to run it when no one is down there (saving you money).
modern direct-vent gas fireplaces with built in circulating fans are quite efficient and pretty much act as a furnace...especially if you get one with a thermostat remote (as we did). 
fireplaces look nice (especially in a basement, where it can get especially dark during the winter months). A nice gas fireplace can certainly be a selling point. 
for the reasons you mention (no need to run new ductwork, no need to replace the existing furnace, etc.)


We have hot-water baseboard heat, so thought that running an entire new circuit in the basement, while nice, would be a challenge and expensive. We felt we might eventually need it (given that we went with bare, stained concrete as the floor) but thought we'd try a fireplace first.

We installed a direct-vent gas insert (we had an old wood fireplace already) with a built in fan. We also added a ceiling fan to the main room when we finished the basement.

When we want to use the basement in January (we're in MN) we just turn on the fireplace and ceiling fan for maybe 10 minutes and the place warms REALLY fast...even the concrete slab. It works amazingly well. Granted, it will all depend on the size and layout of your basement. 

For the most part, we just use it when we are down there (it's the family room, so after dinner TV and the like). If we know it's going to be especially cold for a few weeks, we set the thermostat remote and let it cycle by itself on and off.