The Heartland Championship unions being offered challenges are Lochore Cup winners Wanganui and Meads Cup winners Mid-Canterbury, with Mid-Canterbury having already confirmed their intentions.
Non ITM Cup matches are expected to be played in July, and depending on availability - with at least 14, of this year's probably unavailable for early shield matches because of Super Rugby commitments - Magpies defenders could include some players who will miss selection for the championships matches in August and September.
Mr Bishop said such issues remain to be discussed in full with team management, but it is hoped details of who will be allocated challenges will be clearer by the middle of November.
Hawke's Bay, which in 2013, held the shield for a record-low six days after a first shield match win in 44 years, staved off four challengers this year, ending with a 20-20 draw with Southland, Hawke's Bay's 53rd successful defence of the shield in five reigns as holder dating back to its first shield match win in 1922.
Only Auckland and Canterbury have had more successful defences.
A challenger must win the game to claim the shield, and if successful the new holder's remaining scheduled home matches become shield defences.
Potential 2015, Hawke's Bay challengers Tasman, Wanganui and Mid-Canterbury have not held the Ranfurly Shield.