The monsters would typically stick around the portals they spawn from, and destroying the portal disables it for a while, eventually a new one comes to replace it, with a different theme of monsters coming from it. You might have a portal that spawns Quake1 monsters of various sorts, or one that spawns different Doom3 monsters, for example. Portals that are further from your base have higher level monsters, portals near your base have lower levels, and the enemy base would be home to some sort of big boss monster.
There would be no win or lose condition to this, dying results in respawning back at home base, and encounter spots wouldn't have to be limited to just "destroy the portal". There could be variations on it like defend spot for X minutes as progressively tougher enemies spawn in, or save the NPCs from the monsters, or possibly something that requires you to bring something to the gate to shut it down. All just different takes on the basic idea of kill all the monsters in the area, but with different enemy groups and objectives it make for some interesting options.
Alongside this would be some reforms to the existing RPG system, overhauling the enchantment system (the artificer skill tree would be gone and instead rare, common, or legendary single use enchantment gems would be item drops). There would probably be a removal or reworking of the artifacts in general to make them at most, single use items, but there could be multiple ways to get them, and there would probably be an item type that gives you various bonuses if you have any armor, and would also be subject to the enchantment system. There'd probably be a quite a few changes to the skill system so its a bit more compartmentalized, you'd specialize in one of the four base adrenaline combos (those would have their own trees that could augment one of the adren combos or give you passive bonuses that are in line with the combos themselves) and also specializing in your various "skills" (shotguns, pistols, machine guns, etc., plus medic or engineer abilities). There would also be the potential to have merchants to buy things from, and very possibly some basic quest system in place, who knows.
Damage types could be grouped up into like 5 basic groups, such as fire, electric, chemical, explosion and kinetic, and from there you could really make things interesting by giving various monster types different resistances, or working with generic status effects, making enchantments more specialized so that instead of +12% damage resistance, you might get +26% fire resistance and +13% explosion resistence, there are quite a few options opened up with this.
- Pros:
-Plays more like an MMORPG
-More varied gameplay
-Vastly different leveled players can enjoy the game at their preferred difficulty
-Players are rewarded better loot for exploring and venturing further out
-Loot drops are much more interesting
-Game wouldn't have a lose condition
-Open world, open ended play
-NPCs like merchants and quest givers could be possible
-Lots of options to introduce new content to the game
-Shared world, individual objectives: Players play how they want, team work is optional
Cons:
-Plays more like an MMORPG
-Game would probably be a bit more difficult in general because monsters don't come to you
-Access to powerful enchantments would be much more restricted
-Many RPG elements changed, artificer skills would be non-existant
-There would be less maps available, each one would probably have to be customized by hand
-Map won't end with any win condition, would have to map-vote out
-Enchantments would be less simple, Resistance for example would only apply to certain damage types