99:
This would be fun in teams, as you can try to target players not on your team, or make actions that will prevent a teammate from losing pennies.

Golf:
Averaging out the scores of tablos would make this interesting. It would also ensure that all people get to play for much longer. Could be fun having more than 2 teams for a game.

Farkle:
Not much to say. You can still have high victory scores without shorttening the length of a game. Useless but eh.

Monopoly:
I'm thinking that for this, the players are individuals with their own balance, but they can contribute to teammates' properties. Teammates can also trade each other money at any point, like a normal trade.
So say for example someone owns the light blue group. Anyone on their team can contribute to building houses for the light blues. The owner will get all of the profit, but they can distribute it among the team when needed.
As a consequence of only being a team in loose terms, teammates could choose to be dishonest with their other members. So if someone contributes, they need to make sure they trust the owner of that property. Being on the same team just means a type of insurance if desired, but at the end of the day it doesn't mean much.
Maybe their can be an option so that the remaining members of the surviving team are forced to break apart and go against each other in a normal game. It would be a very long game, but for those who play this a lot I think it would be fun.

Backgammon:
This would be nice so more than 2 people can sit at a table. Players would alternate turns.

Zanzibar:
This would make the token distribution very interesting. Let's say your teammate gets the lowest score, and you get the highest. The tokens should either do nothing, or more likely, they should ignore your teammate's roll and give it to the next lowest roll who wasn't apart of your team.
Team behavior can be an option.

Mexican Train:
Would make the game more interesting as you would have multiple trains other than your own and the public. Can get chaotic very fast.

Duck racing:
This would be a nightmare to develop, but it would also be extremely interesting to see how it changes things. From cards to challenges.

Hello,

Thank you for your suggestions.

For 99, I don't know what to think really.
I see a few issues I'm not sure there are simple solutions for: are tokens shared or everyone has his own tokens ?
If tokens are shared among the team, I'm not sure that the game would be different enough to make teamplay worthwhile, but if they aren't shared, it raises new questions: When someone is eliminated, should the game continue with uneven teams, or the whole team has lost ? Is the final winner a single player or an entire team ?
99 is a rather simple game, so I think that rules for teamplay should stay simple if they were to be implemented, too.

For golf, I don't think it's really interesting if we keep the game as it is. I think we need to add something more in order to make it interesting in teams.
For example, having a way to exchange face up cards from your tableau with those of your teammates, another interactions of that kind, or another way to help your teammates.
The discussion is open what and how exactly we could bring that.

For Farkle, frankly, I don't see the point.

For monopoly either. If you include the possibility for teammates to betray and be dishonest with eachother, what's the point ?
That's already what is done normally all the time, that's all the salt of the game. You can already make temporary alliances and dummy trades to give out all your excess cash and/or make the game last longer if you wish. There is no need to display or manage teams officially.

For backgammon, as well as for chess, connect four and reversi, these games aren't made for it. You can already play in teams unofficially by discussing what to do together. One player actually playing per team, other ones in spectator mode.
Playing like you say will only bring frustations of the kind why did you play this and not that, you are making us lose, You are stupid, I would have done this, etc.
Again I don't see the point.

For Zanzibar, like in Farkle, I don't really see what it would bring. You can't help your teammates, and it would introduce a complicated rule to solve cases like the one you describe for nothing, I think.
Before you make the suggestion, I don't see the pointer for Yahtzee either.
Zanzibar and Farkle are rather simple games, so again, rules shouldn't suddenly become complicated.

Mexican train, here things are getting interesting.
Would playing in the train of your teammate close it if it was open ? Or it would do nothing special. Not simple to answer right now, but it can certainly bring something new to the game. Of your list, mexican train is probably the most promising and the most realistic.

Finally, for duck racing, someone has already posted a quite complete suggestion about it. It would be better to continue discussions there.
I haven't reacted to it because, on one hand it looks quite interesting, but on the other hand, it's going to be pretty hard to set that up. Especially, you can't so easily add or average up challenge results.

I think for 99 people should have their own penies. Maybe the answer is as simple as the following:
, anytime someone on your team gets 33 66 or 99, the other members of the team get an additional penny provided a teammate is still in the game. This way they can keep their teammates from losing the game without giving themselves an advantage, and it gets extended making it an interesting but simple new mechanic not previously possible in free for all.
The team would not suffer any loss if a teammate loses a penny, since everyone has their own.
We could also consider the jack card skipping to the next available teammate rather than skipping the next player, but that might cause problems if one team happens to hold several jacks. It isn't the best idea, but it's still something.

Most of the other games I listed, yeah they are pretty pointless.

Your suggestion for exchanging face up cards in golf is awesome. I really like that. Maybe there can be a separate discard pile for each team and whenever you discard, you can choose if it goes in the general or team discard pile.

Mexicn train:
I was thinking that just like how doubles can ignore closed trains, that same rule would apply for all teammates. In other words, you can play in your teammate's trains whenever you want, but you can only play in an opponents train if it is open. To answer your question, the train would stay open if a teammate plays, since the status is ignored for the team. It can only close if the owner plays in it again.
This way, you and teammates can strategize to get rid of someone's dominos before the other team has a chance to do the same. However, if you mess up a teammate's strat by playing in their train, it is like a friendly fire. So communication will be key in winning this.
As for scoring, only one person on the team needs to play all their dominos to end a round. But instead of the winning team receiving 0 points or adding up everyone's dominos for a score, I think it should take the average for each team. For example instead of adding 30+65+72 points on team 1, it should be that number divided by 3 since there are 3 players.
However this also raises an interesting side effect. The winning team could still have a higher average than other losing teams. So just winning the round as fast as possible will not always be the best strategy if members on the winning team have high values in their hand. I think this is a fair trade off since team members have more trains to deposit their pieces, requiring a different tactic. Also remember that the winning team will still have a slight bonus when calculating their score since one of their players will have 0, making their average lower by default.
The one big drawback to this system is that there wouldn't be much of a reason to use opponents' trains. Perhaps once a player has below a certain amount of dominos say 5 or less, they would not be allowed to use their teammate's trains. Maybe to make it even more harder, teammates would be forbidden from using that member's train as well. This way towards the end, players can not rely on their teammates for everything.

I think we're starting to get somewhere with this, excited to see where it goes from here.

for golf as you said to exchange cards face up from your tablo with your team mates it would be great and that we can do for citedels as well.

So a few of the Scrabble babes have tried this and we loved it. We did it the hard way, but it would be great to be able to play Scrabble in teams. The team members would share a dealt set of tiles and take turns playing them till the end. It was fun to see what each of us would do with the same set of tiles.