There's three alternatives I could think of in relation to a Monster Hunter rep not being the Hunter. Ranked in likeliness:
1) Palico/Prowler. Playable in MHGen/XX, these furry friends have been in the MH franchise for a long time and are pretty much the mascots. Could go for a sillier/cuter MH rep
2) A Monster. Devs in the past have mentioned the Monsters represent the series in cross-overs better than the hunters, so why not here? Brachydios/Tigrex/Zinogre/Nargacuga are believable enough size and move-set wise.
3) Ace Commander/Julius. Pretty much the only NPC I think could be a Smash rep, due to the name leak regarding the MH animated series that has him as the protagonist. He was in the most story based game in the series (4U) as well as GU and has some wicked dual blade skills. However he is ranked last due to the fact that in terms of cross-overs, he would result in a resounding 'who' for most people and would only be recognised by dedicated MH fans
They could always make the Admiral an Incineroar echo for fun?