If you transfer a character to the next expansion:
1. It's often a shame to do so, because he survived here, but might die later, and that would be disappointing.
2. He might get debuffed if the system changes, and that's not very cool.
3. The downside is that you don't use the opportunity to come up with someone new (if you have any ideas, of course). However, if there are few choices during generation, not all ideas can be realized, but at least in terms of personality, someone might encompass more ideas, if not in terms of abilities and races.