
Issues:
One problem with having multiple characters would be that as you got more characters, you would get linearly more powerful; and you would gradually just overpower any monsters that you'd encounter. Having a limit of four allows you to become more powerful in response to monsters, and yet caps the player's potential power.
It is, however, a rather hard cap; and in any games I've played, it really grates against me for how artificial it is. There is no plausible explanation I've ever found that wasn't a cheap excuse for "the game design and/or game engine can't support it".
Solution:
So I'm musing about ways to handle the issue of having a potentially unlimited party. Some, preferably logical penalty needs to come of having a large party. I could see the following being logical:
- Large parties are much more visible, and much more readily attract monsters.
- Large parties are much louder, and more or less ruin attempts at stealth.
- Large parties weigh more, etc, and have trouble with certain puzzles.
- Large parties require more resources, which are sometimes in short supply.
Starting with a tacit assumption that the party would never increase much beyond the maximum number of actual characters in the game, we can make a safe guess that it wouldn't exceed around 20 or so; probably no more than 8-12. With this, we could probably tackle the monster issue by having the number of monsters scale with the player, but scale at a lower rate; such that when you have 12 characters, you have a ... 40% easier time in combat than when you had 4 characters. Enough that it would be worth the bonus of having more characters, but not so much that the game wouldn't be a challenge at all, anymore. Boss battles, I think, would probably scale to not be much easier with more characters - a little bit, though. I'd probably make most late-game boss battles unfeasible to take on without more than a few characters.
For the second issue, players would be able to take major shortcuts through dungeons, if they took a few passes at the dungeon with less than the normal amount of characters. Here, we're assuming that players could leave characters at a tavern in the local town, and then take on the dungeon with less than the usual group.
By doing so, they could possibly sneak past guards, and perhaps open a gate, making it easier for a second pass at the same dungeon, wherein they could take the shortcut that they'd earned by approaching the dungeon with stealth.