I figure if we put Rose in Amy's place, Eleven would still crash land wherever Rose is, and we could have another "case" or "issue" rather than the crack in the wall, since that ended up spanning a season, if not more. As for what Rose and Eleven would do, is up for discussion. When I do DW roleplays, I tend to pick out episodes that have been done already, and tweak them a bit for a new set of characters. So if we wanted to do "The Doctor and Vincent" for example, we could have Rose and Eleven deal with the monster, rather than the canon characters. That's just an example of course, and when we'd get the hang of it, I have no issues making up our own plots or even planets and aliens but for the most part, it's usually easier to use canon episodes until we're feeling comfortable. I'd say it depends who plays Rose and how they want to kick things off, since Eleven doesn't need much altering at the moment.

For HP, it might be a bit of overkill to use both the TT and the potion. Maybe the order would make it a bit less "creepy" in a sense. I figure a potion would be a decent accident, say if someone botched a potion and it was spilled on or drank by Remus, it could work out alright. Again, it would depend on the timing on how we wanted things to flow. If we do something post-war, we'll still need some kind of conflict to keep things on track, it just depends if we want more war/danger or something less hectic. 

I assume we could do both, one doubling as Hermione and Eleven and the other as Remus and Rose?