- the close location of the SH so they won't have to backtrack from the New World to get to Mariejoa,
- the unresolved ending of FI you mentioned,
- the fulfilment of Jimbei's duty to realize Otohime's dream and thus getting rid of the obstacle which prevent him from joining,
- Franky being Tom's disciple as it is quite likely that Tom's family carries the duty of keeping Noah in good repair and Franky would naturally inherit that duty,
- the need for the SH to do something unpreceded on their return to the eyes of the world. Their reunion was a bit underwhelming for the standards Luffy has set,
- oh and Luffy will need a new bounty to reflect his new threat level and Mariejoa would be the perfect stage. He has already invaded all the other strongholds of the WG he won't leave Mariejoa out.
At Mariejoa all those concerns you had could be adressed, besides I feel it's necessary for the Tenryuubito to get some well deserved punishment for their actions. They are an evil which can't be left standing.