At the moment the game I am making can be played strictly as a single player game, but it should work just as well as a multiplayer game and blahblah. I can't really be bothered to explain it all, but the game can be played either single or multiplayer.

Anyway, should I add content that is not feasable to play unless it is multiplayer, even then I am talking about 5-20 people multiplayer.
I'll use an example. An elephant has a challenge rating of 20, these are the biggest and strongest natural animal you will find. A giant has a challenge rating between 30 and 100, 100 is a massive 50ft tall monster that can pick up and throw elephants like they are pebbles. The average person will struggle to beat this sort of monster on their own unless they have stupidly powerful equipment and are prepared.
Anyway, this game is a Roguelike with randomly generated dungeon, but has a lot of unique levels with certain themes and so on. Ranging from ants nests to godly realms and even hell. One of these realms as an example is the realm of dragon gods.
It is a relatively large level that consists of 13 rooms. 5 of those rooms are the rooms of evil dragon gods, 5 of them the rooms of good dragon gods. A dragon gods challenge rating is about 300.
Once you beat all 5 good/evil dragon gods you gain access to the dragon gods of all good/evil dragons. Challenge rating of these two beasts is about 600.
Defeat these two gods and you gain access to the final room, the room of the god of all dragons. Challenge rating? 1500. If this beast was to scale he would be like... 1000 miles tall and is sneeze would be more destructive than a nuclear bomb.

For a better comparison.
This dragon god has 11,000 Hp, deals 900 damage per hit (to someone with no defence), can cast all sorts of spells and has excellent resistances and immunities to everything.
The giant with a challenge rating of 100 has 250 Hp, deals 70 damage per hit (again to someone with 0 defence) and has decent physical resistance.

I'd expect the strongest player to have maybe... 300 Hp and be capable of reducing damage to about 15-20% of what it should be. In short they will never survive more than 2 hits from this dragon god.
Beating this monster with one person is practically impossible.
At minimum you would need 2 tanks just incase the monsters scores a lucky critical hit and kills a tank in one hit. 3 Healers, 2 to pump healing into the current tank and 1 to get any tank that is killed back up and running. And then a whole bunch of damage dealers (the more the better, a single mage cnstantly casting the most powerful spell in the game would kill this beast in about 6 minutes, which would be a more than brutal 6 minutes for the tankers and healers).

Anyway, should the game include content like this? Which would require at least a group of 5-10 people to play it.