(sorry for long post, please bear with me)
Weapon Storage
I'm considering an idea that just occured to me. Originally I planned to have the player visiting the shop often to swap out weapons in order to adapt to the enemies he's facing and to fine-tune his offense. He would have to sell the weapon he didn't want and then buy the weapon he did. Likewise for shields. This would be expensive (since he can't sell it back for as much as it's worth), but the weapons would be fairly cheap.
However, what if when the player buys the weapon, he owns it forever. He can't have it equipped all the time, but he won't have to buy it from the shop again either (unless he wants two or more of the same weapon, for mods or extra ammo).
The player can carry 5 weapons with him into a level, and the rest he must store in a chest or check it in at the weapon shop (they'll store 'em for free). When you get back after finishing a level, or teleport back with a town portal, you can go to the shop and swap out your weapons with ones in storage. This means it's much cheaper for the player to change his strategy; however, the initial purchase of the weapon will cost a lot more than it does now.
Another advantage is that psychotically powerful weapons like the Flak Cannon, Na'mek and Railgun can now be very expensive and hard to buy. Right now I have to keep them cheap so the player could sell them if he didn't need them just then, and then buy them back at a later date. This idea will put a lot more value on weapons that are cheaper, and makes the more powerful weapons a better buy.
(all of the above also goes for shields)
This also gives a new role to MODs. Now that you own your weapon forever, you might still want to occasionally buy mods for it to change its function slightly. You can always pay a little to change it back, and it's cheaper to do so than to sell the modified weapon and buy back the original.
So, you own your crossbow for life. At some point you might want to buy the Fire mod for it, and later you might change it to Poison, and then later you're fighting monsters that you can't poison so you change to normal arrows, which are cheaper to buy and fire faster anyway.
Ye Olde RPG Elements
I also want to make it so that you have to level up a weapon in order to make various mods or other weapons appear. Leveling up the Crossbow by attacking enemies with it will open up the Fire Arrow mod, Poison Arrow, Piercing Arrow, and also unlock the Stake Launcher weapon at some time.
Leveling up the Green Standard armor will make Yellow Standard available, and leveling Yellow finally unlocks Red. It also unlocks Strength Shields, which must be leveled up to reach their full potential. Leveling up Precision armor increases the maximum bonus you can buy for it.
I don't want to take this leveling thing too far, but I've discussed the idea with my friend and I think it will help give every weapon and shield a place in the mod, and encourage the player to explore weapons they might otherwise ignore. It will also help balance the mod, so that the player can't just save up his money and instantly jump to a super-powerful shield early on.
I also someday want to make it so that the storyline places a limit on what's available in the shop too, so the player has to progress through the game before the stake launcher comes into stock at the shop (even though the player has already unlocked it). Performing some sidequests will reward the player with a weapon, modded weapon or shield that wouldn't be available yet otherwise.
So, what does everyone think of:
A) Making weapons and shields expensive, but letting the player store them forever instead of having to sell them back and forth
B) Introducing RPG elements such as leveling up weapons and shields to increase their damage and unlock more mods/weapons
Thanks, all!