Having said that I dont think the tiste are un-playable (i have always thought of them as malazan equivelent of elves) You could simply play them as elven rangers, you can take the warrior or mage path depending on preference.
THe tiste arent all powerful, bear in mind the battles in MT.
As for soletaken/divers you could simply play them like mages with access to certain spells. the abiltiy to change into either a single powerful creature or a swarm. If you remember the chase in HOC with Karsa and the divers it is mentioned (i think by mappo) that he had gotten more powerful and could create more duplicates. SO as you level up you can add more swarms or more powerful creatures. Start out as a wolf become a dragon when your really high. Or start out as some rats and then maybe end up as a pack of wolves.
I would, however, make it possible to A) utilise other abilities as well as soletaken (so you can still become a fighter or mage)and
![B)](https://forum.malazanempire.com/public/style_emoticons/Malazan/cool.gif)
THe main question for shape-shifting should be how the divers fight. can you control each creature or swarm as an individual or do they stay together.
hope i havent shown my extreme novice status with this ramble.