Adding RPG mechanics into an adventure game does come with a big cost/benefit ratio. There is a reason why RPG's take many people and many many years to make - every single system is intricate and complex, and has a HUGE knockon effect. The only way its really worth it (financially) is if you can get people who are interested in RPG's (probably one of the largest most lucrative genres out there) interested in your game. Unfortunately what tends to happen is that those RPG players see the mechanics being super light, and adventure game players see the mechanics being extremely intrusive.
It certainly CAN be done - UNAVOWED is a perfect example of this - but there are sacrifices that have to be made. Languages is a BIG one. Writing is cheap - but translation is EXPENSIVE. If we want to keep our games profitable, we have to translate into multiple languages. Having classes or different characters that can take different paths, while still keeping the 'adventure game' feel of a tight character driven narrative starts to get damned expensive. Each character you add sends your text budget up an order of magnitude.
Then you have testing. We test our own games, and every time we do a change we play through the game from the beginning to the end. Multiple characters can take that from a 4 hours speed run to an 18 hour speed run. So your testing and release schedule for bug fixes can jump VERY easily.
Again, this is built into an RPG, where you can charge a higher price point because people WILL try to play through the game more than once. But for a linear narrative adventure game, the prospect of multiple playthroughs at a lower price point just doesn't justify the cost you have to put in.
If we had a larger team, or ran multiple projects I can definitely see us overcoming lots of these issues - but with our goal to remain a 2 man team, it's something that will require a lot more planning and experience on our side to get right.