I am wondering why the Trill characters were limited so much in terms of being joined, and in their number of past hosts. Under CODA, Trill characters must be joined and have 1 to 3 previous hosts. However, under the old ICON system, they could be unjoined; and if joined, they could be the first host, or have a greater number of past hosts (though that system also recommended 3 or less).
Not allowing for unjoined Trill precludes the majority of the species from being used. Unjoined Trill are obviously in Starfleet, as Ezri was unjoined before being forced to except the Dax symbiote. After her joining presents another problem; by that point Dax had something like 8 previous hosts (I guessing here), which can't be accounted for under the rules either.
My guess would be that the potential strength of the joined species ability is why the limitations were placed. A large number of previous hosts makes the joined ability fairly powerful, and also makes being the first host virtually useless. And if you tried to make an unjoined Trill by simply excluding the joined ability, they would be left with only the Skilled species ability, making them under powered compared to other species.
I am considering a 'house rule' alteration for determining number of previous host to use a new formula. Roll 2d6 and allow the roll again on 6's rule. Take the total of the die rolls, divide by 3, and round down to get the number of past hosts. This allows for being the first host (you would have to roll 2 one's to get that result), and would usually result in 2 previous hosts (2d6 yielding a 7 result on average) but would allow for a higher number of previous hosts (you would need to roll at least four 6's to get Dax's number of previous hosts). I think this will probably be workable.
I don't know what to do about unjoined Trill however. I am considering an extra courage point (and ICON gave them +1 courage, so it isn't completely out of line), but this would probably apply to both joined and unjoined Trill.
Any ideas/comments?