I can't imagine that no new summons will be implemented for level 100.
A summon has been added in every expsansion (if we exclude HW's Bahamut trance)

HW: Bahamut Trance
SB: Demi-Bahamut
ShB: Demi-Phoenix
EW: Ifrit / Titan / Garuda

so I can deduce that something will be added in this case too... not just upgrades to existing spells