To point it out again, if, that's a big "IF", China manages to capture Sikkim & chokes the chicken's neck, only viable military option India will be left with is invading northern areas of Bangladesh.
Bangladesh holds strategic landing grounds both side of Brahmaputra's southern journey. Go to Google map & see how the river flows between Kurigram & Roumari. India for a long time had eyed Roumari, tried to put it up internationally as disputed area by attacking the area preemptively & we had even fought brief border wars over it (see Battle of Roumari).
But then again it's extremely unlikely scenario that China will ever go all out war with India. Things these days don't work that way.