What's really nice about this is that with the update that brings these characters in, they made some substantial changes to the summoning system. They killed the summonable Wyrmprints and moved them to the shop, which was the biggest complaint, upped the amount of Eldwater you get to use to promote characters with, are giving out a huge amount of compensation Eldwater for previous summons to all players, and slashed the summoning requirements by a fifth. They made the summon system a lot more inviting to all players, which is a perfect way to lead in to the crossover since many new players may want to check out the game with it.
As for the Crossover itself, I'm looking forward to the new mode. It looks like a tower of defense type thing, which with DL's gameplay should be fun to play. I also am pleasantly surprised Marth was chosen for the game since I thought they'd stick with the OCs, but getting him is a nice nod to the rest of the series as well as giving him some love considering his bad luck in Heroes. Alfonse and Veronica are also great to see, though I'm curious how they'll implement the latter since there's not a proper mage class in the game.