What Drangir said on all counts:
- Weapon swaps are super-easy. Simple spears are pretty easy as weapon swaps go, just trim off the old weapon, use a pin-vice and fine drill-bits of appropriate size to drill through the fist a hole big enough to slip the spear shaft through, gently insert the spear.
- Spear shafts are easy to make from styrene rods, if not wire. It's the spear points and pole-arm blades that are tricky to source, unless you have a bunch of spares from other kits... if the contents of my bitz-boxes are anything to go by, spear points tend to be easier for me to source than things like the countless flavors of poleaxe and halberd blades.

I think there's probably a market for plastic fortification terrain kits for things like gambions, sharpened posts, stakes, and chevaux de frise. I've gotten a few such bits from somewhere before, probably leftovers frome someone's Warhammer/40K stuff, which I've never put to use, or maybe from some Mantic Terrain Crates or Dungeons & Lasers or something, and really, it's the sort of thing that Dungeons & Lasers and the Terrain Crates specialize in - scatter terrain for skirmish and wargaming use. A box of, say, four or five identical frames of the bits for these sorts of defense works, would be pretty easy to do, and would surely find a use in a variety of gaming and dioramas in 28mm and similar scales.
A supplemental set in the same vein for things like sandbags, barbed wire fencing stakes, hedgehogs, and dragon's teeth is an obvious choice to suit Trench Crusaders and WWI/WWII gamers, and maybe something similar in a urban crowd-control flavor for the Zombie Apocalypse with Jersey barriers, steel fences, orange construction cones and barrels, and that sort of thing would find some post-apocalypse, pulp and sci-fi buyers as well.
Scale model manufacturers used to make specialty kits of this sort - usually in 1/72 scale, which is visibly too small for 28mm purposes - but I think a lot of those kits have been out of print for years. The kits used to include two or maybe four pieces each of things like sandbag walls, chevaux de frise, and gambions, with maybe a smattering of wooden barrels, signposts, ammo crates, undersized pup-tents, or whatever. Sometimes you'd get a guard shack and blockade gate, small foot-bridge, or watch-tower instead of the barricades, and sometimes you'd see a WWII-focused kit with steel drums, telegraph poles, brick walls and steel hedgehogs instead of their wooden Napoleonic counterparts.

One of the dozen (give or take) classic 1/72 Italeri Battlefield Accessory sets from back in the day....
That's maybe a little two all-purpose to be much use for proper wargaming territory - better suited for a small diorama - and a modern 28mm set might best focus on the things that are maybe a little harder to find these days in a world where modern specialty kits sell steel drums and wooden barrels by the bucket-load, and 3D-printing and specialty terrain kits fill a lot of the void for things like bridges and small buildings.
I think a 28mm kit focused on maybe three to four different types of barricades with enough for 4 to 8 such items, with filler like signs or whatever if possible, is maybe the sweet spot for this scale, and target audience of skirmish and tabletop gamers.
But, that's what I imagine - what would suit your gaming purposes better?