I suppose it's really a question of patience and fortitude in the long term. Was there any luck with porting the Manaketes, Taguel, Female Tactician classes and certain unique character textures like Cordelia's and Sumia's Pegasus Knight models? I've seen someone improvise a Thief by removing the hood from a generic Outlaw and give it the Master Ninja's sword animations. The idea was to make Gaius (and by extension for other classes, Cordelia and Tharja) and overall, it wasn't too bad, but that's been made as DLC, much like DeathChaos25's Chrom, Frederick, Lissa etc.
For some of the characters/classes, I assume it'd be appropriate to fill in missing animations with stock animations from Fates. If the Assassin and Trickster you've pulled come with their animation sets (which I assume they do) Gaius could be among the easiest of characters to port.
I'm sure it's probably more difficult than I'm wording it but it'd certainly be interesting to see how far it can go, right?