I really like Magic Guard Celebi, it may be immune to Leech Seed and stuff, but I now see two amazing roles for it: U-turn / Baton Pass and Life Orb. It now is the only Pokemon who does not take entry hazard damage and has access to U-turn, which is invaluable for VoltTurn teams. Baton Pass plays a similar role, as it is not worn down now and constantly QuickPass. Then we have Life Orb. Magic Guard negates Life Orb, which is really cool. You can now run Nasty Plot / Energy Ball / Psyshock / Earth Power for a really good offensive Pokemon. A thing people have yet to mention is how hard it is to wear Magic Guard Celebi down, as now Toxic stalling is not an option and Recover greatly increases its longevity, making you forced to attack it. A cool set I think could work would be a Calm Mind set, with an EV Spread of 252 / 84 Def+ / 172 Speed with the moveset consisting of Calm Mind / Recover / Giga Drain / Earth Power, the EVs to be as physically bulky as possible and to outspeed max Speed Heatran. It could use some tweaking, but it might be cool. Overall, my vote goes to Magic Guard Celebi this round!
As for the other Theorymons, Volt Absorb Jellicent does not make sense flavor-wise, I don't like it very much. Grass / Poison Telligent is cool, but I don't think we really need to make Tangrowth harder to take down and it seems like a way too good of a buff. And Grass / Dragon Sceptile is just weird, I don't like Sceptile so bias, but I don't think Sceptile would make an impact on the metagame at all.