There will be
- a resurrection dragon (old dragon come back) in week 3.
- another legendary dragon and a rider in week 6.
- a tower branch in week 8.
- a festive dragon branch in week 9.
- several one-week branches offering exotic runes, unknown when
And then there’s a trading post, which so far has appeared twice a season. It stays for 5 days and offers items of varying quality for sigils and sometimes for other currencies too. Each offer is up for one day.
As a beginning player, the tower line in week 8 is probably good to get some of, it will boost your level progression a fair bit with the speed-ups it contains. Getting a second dragon is probably nice as well, but best wait until at least week 9 to find out which dragon looks best for you.