Hmmm, so many choices....
First thing, I'd get a 30" Stan Baker Big Bore bbl for my trap 870, and get some open tubes for it for skeet, 5 stand, SC and whatever, even dove shoots.
Second thing, run around buying classic 870s, and setting up one for every conceivable use. Got a good start on this already(G)...
Third, get a nice classic SXS, a Fox etc, and set it up as a quail gun. This would be a light 12 or 16, about 6 lbs, and pretty.
And if I just HAD to scratch the sub gauge itch, a 28, possibly the Ruger, probably the 870 WM in that tiny bore.