I have a couple of ideas for multiplayer, as well as some questions.
I always think RPGs are more enjoyable if you find someone to play along with. They're not games you can sit down and play for "a bit". You literally need to sit down and get into it, which is a lot more than you can say for your average FPS game. However, there are a lot of ways to make multiplayer more accessible, to allow players to hop in and play for 30 minutes then logging off after.
Things like matches are not exactly something you find an awful lot in RPGs. The closest thing to a match in an RPG would have to be some sort of large scale PvP event in an MMO. I think, that with the realistic, tactical and not to forget fun combat system, various sorts of matches would work very well. Perhaps a Free For All in a Colosseum-style arena, where you get to customise your weapons and armour, and choose the best tools by having a look at what everyone is using.
Gamemodes that I think would fit nicely, if tailored to work:
Team Deathmatch & Free For All
Simple, good ole' versions of TDM and FFA. Teams could end up being quite big, if the maps support it. Imagine Battlefield-style open levels, like a city. Your team of 8 starts on one end, the other team on the other. You can literally charge straight through the main streets like a bulldozer, swinging left and right with a giant maul as you go head-first into a crowd of enemies, or you could be tactical and divide your team into 4 squads of 2, and split into all the alleyways, setting up an ambush for the unwary. This would definitely work, especially using options like turning Friendly Fire off, or turn off special types of weapons to encourage people to try something else (Turning off all blunt weapons, turning off thaumaturgic powers as a whole or separate powers, or giving everyone a standard kit of weapons and armour to level the playing field.). The maps for FFA would be significantly smaller, and allow for the same customisation and options.
Siege/Capture The Objective
Setting up levels with a short story that introduces you to two sides of a conflict. A siege would obviously be one team defending their castle from the other, with sub-objectives like "Capture the Eastern Gate" and "Capture the Throne Room". Castles would be outfitted with their normal defence mechanisms like murder holes, fire pits for archers to light their arrows and more.
Capture The Objective would be a bit different. You get thrown into a situation that is pre-made, with a short story to describe the position you're in and the objective both teams have. A good example would be something along these lines:
"Your convoy is tired. Charged with the security of a rich and influential Lord's treasury, you're transporting a chest full of gold and other valuables. The night is cold and dark; the rhythmic drum of hooves the only thing keeping you awake. Suddenly, somewhere off the road, a rustling of leaves. A thunderous sound becomes more and more audible, as twigs crack and a silhouette becomes visible in the darkness. Out of nowhere, a giant boulder shoots out of the foliage, narrowly avoiding you as you dive out of the way. With a loud crash, the boulder obliterates the carriage you were charged to protect. In the dark you can hear battle-cries, as arrows whiz past you and armed men appear out of the cover of the trees."
This situation would drop you on the side defending the convoy. A carriage is crashed on the middle of the road, the chest on the ground, there for the taking. Enemy players are coming out of nowhere, inferiorly armed but great in number, making dashes to take this chest to their camp nearby. Should they succeed in doing so, they will get the objective to defend their camp, and the convoy to reclaim the chest from there. If they do not succeed in taking the chest from the wreckage though, the objective for the convoy will stay for them to protect the carriage.
The other side would obviously have a different story, but the biggest difference would be your kit. The convoy is heavily armed, whereas the ambushers are not. The balance would be 3 against 6, or 4 against 5. You get to customise your weapons a little bit, so you'll be able to get a good variety of weaponry against your opponents.
This is a nice way of setting up small stories in a competitive environment, and giving the players some interesting scenarios to delve into!
Duel
Probably the most obvious competitive gamemode for an RPG. You choose your kit and just battle it out one on one with whatever weapons and armour you choose. Would have arenas that are not very large, perhaps with some environmental danger to them (Fire, falling, etc.). Maybe the ability to set up a tournament with custom sizes and options like weapons, armour and thaumaturgy. You could create a tournament where nobody has any armour, and has a weapon that could literally kill with a single blow. Alternatively, you can set up a tournament where you have very little else than your thaumaturgic powers. Maybe some leaderboards, a lobby system where you get to invite someone to a duel and lots of customisation options.
Co-Op Campaign
Obviously the most important gamemode. You can invite a friend to your world, and they'll join at your position or alternatively where they were when they left last session. Just like NPCs will react to your deeds, they will react to your partner's also. A city guard may tell you he thinks you're a lot more noble than your friend, and that you could teach him a thing or two about manners. Some children might tell your friend that they think he's probably the stronger of the two, and could defeat him in battle easily.
With a dynamic world and reputation/relation system, you can get a lot of cool dynamics going, like "The Young Knight & The Thief" or "The Old Thaumaturge & The Mercenary". You will feel compelled to talk to townsfolk more and explore, just to see what people have to say about you and your companion, and to work together on solving problems with the skills your character has developed and is praised for by the world around them!
I understand that most Multiplayer Gamemodes do not fit with the overall theme of the main game. I think though, and this has been said more often, that Sui Generis is a fairly obscure game. It's something you either adore or despise, clear and set on what it is and pure. I think though, that with a combat system and physics as realistic and hilarious as this one, you will want to share that experience with others in small bursts. Not everyone likes taking 3 hours out of their day to get ahead in a big questline.
A lot of RPGs have done this right. Mount & Blade is one of the best examples. While clearly not the strong suit of the games, the mechanics make for an interesting fight against actual people. The same goes for games like Chivalry and War Of The Roses, even though those are in their very nature competitive games.
Tl;dr
There is a lot of potential to make competitive gamemodes something that compliments the game rather than weighs it down. In any tactical game, you wonder how you would do against a friend rather than the AI, which I feel goes with Sui Generis very well.
Just dream the humongous difference if DS ever had FF off between summons and summoners. Summoner would had to think for the second time, or third even fourth since there would be world of differences in invasions.
