Technically, it should be possible. Here's my idea:
1) Player character. Instead of Eddie, the player gets to choose one of the non-hero human npcs:
Headbanger;
Razor Girl;
Thunderhog;
Fire Baron;
Roadie;
Zaulia;
Grave Digger;
Possibly Kabbage Boy band-members.
I figure these should be the most fitting because they can use the same animations as Eddie (+ the would have the ability to manipulate objects without looking too wonky) or have their own, and have basic customization options.
2) Customization. Once the character model is selected, the player would get the option to customize it by choosing:
Skin\hair\eyes\clothes colours (when applicable);
Hairstyle - for example Headbangers should have standard or glam options; Grave Diggers have several options (messy, downed mohawk, etc.);
Additional clothes - for instance Headbangers should be able to choose a beanie, a helmet, a bandana, a pink cowboy hat, etc., a vest or t-shirt, and so on (for Headbangers, Razor Girls, Thunderhogs, and Roadies, the Hair Metal Militia customization options should be available).
3) Gear. Once the character's model and appearance are selected, the player should have the option to select their weapon and instrument. I think the following would do (note that ideally, they should not have buffs, but I suppose it would be possible to make buffs work as a kind of purchasable enchantment):
Weapons:
The Separator (Eddie's axe);
All vanilla options for the axe (electric, sharp, bloody, etc.)(aesthetic only);
Razor Girls' shotgun (may or may not shoot);
Zaulia spear;
One of Ophelia's blades;
Lars' sword;
Lita's polearm;
Grave Digger's shovel;
Punishing Party's spear;
Doviculus's polearm;
Reaper's scythe;
Druid's sword;
Simple drum stick (possibly);
The Hunter's weapon;
Meat (as seen in some cutscenes)(more of a joke weapon).
Instruments:
Clementine (Eddie's guitar);
All vanilla "strings" for Clementine (Brawlers, High Voltage, etc.)(likewise, purely aesthetic; when applicable);
Bass guitar;
Lionwhyte's microphone;
Hydra (Doviculus' guitar);
Six-Stringed Sorrow (Drowned Ophelia's guitar).
4) Vehicle. One is needed to cover the distance, of course. The player gets a vehicle-spawn solo at game-start, but the vehicle should be chosen during character customization. Available should be most human vehicles.
The Deuce (Eddie's car);
Headsplitter;
Hairsplitter;
Hearse (Drowned Ophelia's car);
Chariot (Doviculus' car);
Thunderhog trike (minus Thunderhog);
Glamhog trike (minus Glamhog);
Fire Baron bike (minus Fire Baron);
Squealer (Fletus' car, minus Fletus);
Lightning Rod vehicle (possibly);
Mine cart (like the ones seen in and around Lionwhyte's mine).
5) Quests. Seeing as how playing through the original questline as anyone but Eddie would not make sense, it would obviously have to be removed. Hence why I said 'sandbox'. Instead, the player should have radiant quests, similar to those the Hunter gives, but with more variety.
Hunt - kill x number of whatever beasts;
Berserk - kill x number of whatever enemy units that spawn in your immediate vicinity until the time runs out (think Rampage missions in GTA)(all spawning units in a quest are the same - for example parties of 4-6 Druids, or Hairbangers, or Grave Diggers, you get the idea);
Race - self-explanatory: vanilla racing but should be able to have opponents other than Fletus (any vehicle npc in fact), and\ or up to four opponents;
Ambush - vanilla ambush mission;
Solo competition - play random solos against another npc. Whoever plays all the solos better (or perfectly) wins. (could be complicated to implement though);
Mounted race - same as vanilla racing, but player has to first catch a specific type of animal and use them as a vehicle (could be complicated to implement);
Recover strings - go to the metal spiders' lair and kill x number of spiders;
Get amps - go to the Screaming Wall and help the Roadies bring amps (basically repeating the vanilla quest 'Pilgrimage of Screams'.
Anything else, if possible.
And the quest-givers should spawn randomly in random places, or in set places but once in a few in-game days, etc. As reward, they give you Tribute points.
6) Shop. Driving inside Forges doesn't make sense for characters other than Eddie, so instead, the player should have some alternate way to activate them, or maybe they should have a dedicated merchant npc? Purchasables should be:
Instrument (replaces the current one);
Weapon (replaces the current one);
Vehicle (replaces the current one);
Character customization options (clothing skins, hairstyles, clothes - see point #1);
Weapon buff (possibly) - should act as the axe/ guitar improvements in vanilla, but instead work with any of the weapons/ instruments. It's a matter of adding the effect and shader (or just the effect if it's too much of a hassle to add the electric shader to Lita's polearm, for example).
7) Other stuff. Surely, if anyone is able to implement even a fraction of what's listed here, other stuff's bound to appear eventually. The sky's the limit. And while I'd really love to see this become reality, I fear that it won't, since it hasn't so far. Regardless, a man can dream.
What do you think?