I don't think 'that jutsu' will be FTG, whatever level at that, simply because FTG doesn't require a lot of chakras.
Now here are the facts and my predictions about FTG:
1. Nobody so far in the manga mastered FTG except for Minato (fact)
2. Because Minato mastered it, it surely doesn't need gigantic chakras. (I think it's a fact)
3. If FTG is frog's jutsu and doesn't require gigantic chakras then surely Jiraiya should be able to master it, in fact Jiraiya should be the one to teach Minato if that's the case. (common sense)
4. If FTG isn't frog's jutsu and doesn't require gigantic chakras, then why Jiraiya and Kakhasi aren't capable of doing FTG (common sense). They both are able to do rasengan after all. (fact)
5. The possible explaination is FTG is a part of space/time jutsu which is a bloodline limit. That's why only Minato can do it. (prediction)
6. All of sudden Kishi mentioned the 2nd has space/time jutsu as well. (fact)
7. If space/time jutsu is bloodline limit then Minato could be the grandson or great grandson of the 2nd. (prediction)
8. If space/time jutsu is not 'that jutsu' then Naruto will learn 2 new jutsus. (common sense)