Gordon would be extremley hype. Half Life is as iconic as many series in Smash, and a lot of the sci fi weapons and monsters could lend to a great moveset. The crobar, gravity gun, gauss cannon, snarks, and barnacles (using them like in Opposing Force) could all make for a very fun and unique moveset. For stages, they could go for Black Mesa, Xen, Ravenholm, or even the Citadel for good options. Spirits can be all of the aliens, and Mii costumes can include combine overwatch armor, HEV suit, Barneys guard uniform and a headcrab hat. Remixes can include Hazardous Environments (the song that plays when every valve game boots up), Nuclear Mission Jam and Path of the Borealis. I dont see it happening this dlc pack, but if we got a second pack Id consider him (and a valve character in general) much more likely.
And also, Nintnedo tried to hire Valve to develop a half life game for switch a few years back. Nothing came of it (besides a cancelled Half Life RTS) but it goes to show Nintendo KNOWS how big a deal this series is.