As it sits now, the DDO holiday event calendar has a big gap in the middle of the year. We start the calendar year with the Midwinter Festival/ Risia Ice Games in mid January, then the Anniversary Party at end of Febuary, sometimes the Traveler and Degenev Brothers show up as spring begins, around Easter we have Crystal Cove opening for a bit then nothing until Cove sometimes make a replay around Talk like a Pirate Day in Sept, otherwise its nothing all summer/fall until October for the Night Revels, then Festivult in Dec until the end of the year.
I personally would like to see an event in summer similar to the Midwinter/ Risia event to the effect that we could add electric/acid/sonic resistance and magnetism/corrosion/resonance spell power to our gear (Midwinter has fire/cold res & glaciation) and add elemental damage to our random gen weapons, up to 'burst' level (acid burst etc..) like we can with icy damage from Risia.
How the events are implemented, to me, are not as important as the rewards we can play for. IMO the Risia-like event could be a similar to Crystal Cove in it being like a wilderness open instance wherein we kills mobs for tokens, which we can then turn in for random recipes to craft on the extra damage. The Midwinter-like event could be similar to Mabar with a set of reworked quests that offer quest specific materials that can be used to exchange at a vendor for the resistance/ spell power buffs.