FTG could be a blood line limit but its not. You say if it wasnt, wouldnt other jounin be able to do it. How about talent and skill, just becaue your a shinobi doesnt mean you are capable of doing all kinds of techniques. Its like the rasengan, only 4 that we have seen can do it. Jman, the 4th, Kakashi and Naruto. So just because they are the only 4 that can do it, does that mean that the Rasengan is a blood line limit????? Its obvious that its not... Naruto was able to improve the rasengan, somthing that the 4th couldnt do, maybe because in that department Naruto had more creativity to make it happen and in that department Naruto was more talented and skilled... Also goes with Naruto's motto of never giving up. How about Sage mode, Naruto was more talented and skillfull to be able to master Sage mode at a higher level than Jman. So sage mode must be a blood line lmit too since not all the other jounin are doing it, right???? WRONG
Please read carefully. I said "If it's not blood line limit, then surely all the jounin would be capable of doing it to some degree." My "to some degree" is similar to your "talent and skill" concept I think.
So far we know 5 people that can do Rasengan not 4. You missed Konohamaru. Now Rasengan is an A-rank jutsu, it's a shape manipulation techniques, but the reason not a lot of people can do it is mainly because it's not taught widely. It's very hard to master, because before Naruto, you have to have excellent chakra control to master it. Naruto use a clone to do Rasengan, that way you don't need excellent chakra control, hence Naruto and Konohamaru are able to do it.
Sage mode is even more picky than Rasengan, you need a teacher with you all the time at first and then you have to learn it in mount frog (at least if you want to learn it fast). Even then if you are too successful, you turn into a stone frog. The fact is sage mode is not blood line limit, everyone could learn it, you don't need certain blood line to do it. The frog stones in mount frog are the prove.
Just because some jutsu only mastered by few people doesn't make it a blood line limit jutsu. Blood line limit jutsu is a jutsu that can only be performed by someone with specific blood line. Now the manga told us that the 2nd, 4th and Tobi all has space/time jutsu. If the space/time jutsu is not blood line limit, please think the points below:
1. Logically the 4th learnt it from Jiraiya.
2. Surely Kakashi the copy ninja would be able to do it.
3. It won't hurt Konoha to have a lot of shinobis with space/time jutsu.
4. Surely all the Konoha's badasses should be able to do it, the sannins, the 3rd, Danzo, Itachi, etc.
5. With enough people (say 5 at the same time) mastered the space/time jutsu then Konoha would be unbeatable. Look what a trouble Tobi is. Look what Minato capable of, sending the 9 tails attack elsewhere.
You could have all the creativity in the world, you may have all the talents in the world, you might have never gave up attitude all you like, but if it's a blood line limit jutsu and you don't have the blood line part, then you WON'T MASTERED that jutsu.