Question

I think Organising a conference.... be an excellent idea

Answer 1

the answer is 'is'

hope it helps  u

Answer 2

There may be many valid auxiliary verbs in the given blank which can be put up in correspondence with the gap in the aforesaid.  These words can be considered to be an essential fraction of the TIV (tense indicating verbs)  these words indicate the tense of the sentence and also point towards any present emphases. Some words which can replace the gap are would,  will,  shall,  should , can,  could, may and might.