I like the idea however i might as well inform you of it not being very likely unless another project decide's to hand over their 2.4.3 source code and both teams working together being a very unlikely case. If Elysium had both 1.12.1 and 2.4.3 servers already with both having dev teams working then yes otherwise they would have to start on a 2.4.3 from scratch and risk release having many exploits.
Only very few projects have the potential for expansion so far i only know of two with the option due to having 1.12.1 and 2.4.3 for doing so and that would be wow-one and kronos but between those two i would consider kronos more as they have full logs on everything included with armory ensuring that they know how to access player information along with raid and display it or it could be used as evidence if they decided to use the expansion idea should you be missing an item you could prove it with raid logs provided from the server itself.
note to mention no server has yet confirmed if they ever plan to or are considering such even if i agree with you and it would be nice with a natural expansion kinda upgrade after naxx is done on a 1.12.1 server.
But then again no server has yet ever done the expansion thing which is partly becourse most only focus one and few have multiple developer crews for different expansions and vanilla is a long expansion so it is something that can first happen long after Naxx is out and cleared. not to mention there are more fun servers around than vanilla ones like elysium, wow-one, cresfall, kronos etc. and nostalrius fell so it cannot be added to the list while wow-one i think has suffered from low population for awhile and kronos is a medium/high and elysium takes the cake with a very high population beating all competitors at least for now but let us wait a month or two to see how much the corruption affects population as many still do not yet know it.
anyways long story short: Elysium is unlikely to have an expansion especially since they are even lacking developers for one version being 1.12.1 and 2.4.3 not being spoken of but well they could be extremely lucky and a 2.4.3 project joins them even with the current corruption being known