IMHO,
- The MI6 guy was excess bagage. The idea was that he was to take over the "international spy" portfolio from Ziva. But the show has not done much international spy plots, and in regular crime plots, he is just another guy.
- The Abby character will get a mention now and again, as does Tony, but she is gone. It is clear that, she has some sort of personal problem with MH and they had to spend the entire year with an artificial scenario where they never appeared together; and also she has enough money. You may see her as a guest on one of the other two shows in a few years. That is all.
- They have enough characters going forward. Do not look for any replacements. The "spy" part of the MI6 character will just be forgotten about as it appears that that element of the show has been abandoned. The stuff Abby did will be split between Palmer doing the science and McGee doing the computer geek stuff, with references to an unseen lab rat character sending up results from the basement but never a part of the "team".
- The show will continue as long as MH wants it to, it is all about him. When he decides to quit, CBS will be faced with the same choice it had with JAG or Fox had with the X-Files. They could have, but did not, continued the show with the new characters, which is how things go in real life, or ended it. Who knows.