not sure if this has been discussed before
with a set-able time frame and aditional simple options

pretty bad if your topic is bumped and you are not online

No - Because it's just spam. No need to bump a topic if you're not even online.

And if all had auto bump on their topic, then your topic would end up on page 7 in 30 seconds.

No, if you're not online when it is bumped the guy could be stuck waiting

also the topic, although tracked still, could be out of date or the items already sold since they may not be on top of things

the only reason to bump offline is to get to the item you have been looking for the past 3 months while you can physically bump once a day or less