If I can express my opinion, I agree with Omnirizon. Giving a good money prize and some gems (opportunity to diversify) to the winning nation would be IMHO the easiest and less unbalancing way to make the Arena match worthier.
Of course the ideas about random items and summons are interesting but maybe way more unbalancing - making the arena prize a powerful artifact seems unwise, some nations with natural early access to thugs (Fomoria? Niefel? Hinnom?) could win the first Arena Deathmatch and become even more powerful (Fomoria! Niefel! Hinnom!).
Also, forge bonuses and increase in magic paths seem to much more benefit magic users, which seems like a contradiction in terms for the Arena which should be the place for the best fighters of the nations to battle, regardless if they are mages, melee warriors, ranged fighters. (For that, I already see the trident like a quite weird thing, a crown or helmet would have been imho more appropriate, expecially for i.e. someone who wins with the Ethereal Crossbow and now can't win with that no more
)