Think you'll keep hold of Bell and Karius?
In the winter transfer window? Yes, definitely.
In the summer and beyond... who knows? I think that while Bell is good, he's not
that good, so I'm not sure a bigger/richer club will be interested and pay our asking price. Karius, on the other hand, is a fantastic goalkeeper and will almost definitely make the step up in the near future. However, from what he has said, he's pretty happy in Mainz and might stay for a bit longer. A transfer is more likely to happen in 2017 than 2016 imo.
I have to say, I'm not too worried losing either of them, the replacements are there if they chose to leave. We have a young CB in Alexander Hack, who has done really well for our U23s in the third division and the last two games with the first team where he replaced the injured Bungert. On the ball he is on a different level to all our other CBs, really excited about him.
The goalkeeper situation is a bit different, not sure if any of our backups are immediately ready for the step up, but we have a really talented generation of keepers coming up through our youth teams and U23s. For that reason, I'd like to have Karius for a bit longer, so that his replacement is ready when the time comes, whoever of them it turns out to be.