A simple second clan invasion of the IS would be boring, imho. Not to mention that a power that is capable of smashing through the Falcons, Bears, Dracs and Wolves - i.e. some of the most powerful IS militaries - would probably inspire yet another IS alliance against it.
I would like the opposite better: Bastion clans invaded by a brutal IS force. The invasion fails horribly, and justifies a counterstrike which does not unite the IS because only the invaders (and some nearby) are attacked.
However, overall I prefer a disconnect for a while. Let them have some sporadic contacts at max. The clan worlds should not be worth invading (not with that level of resistance to be expected), and the IS should be too strong to invade.