Yeah, lvX would definitely fit better, considering it's almost exactly how Mega Evolution works in the game (which makes it kind of odd for the TCG to have picked the rule they did). It's also one of the more fun gimmicks in my opinion, since instead of balancing power with drawbacks (which is problematic in the case of EX because the drawback comes far later, if at all) it continued to scale power with how hard it is to play (which is how almost every other TCG does it). LvX and Legends (1 Pokémon per set, not like the double dogs) were the only over-powered stuff I could really get behind for this reason.
If you do copy the lvX mechanic, it might be best to add the same-name clause as well, otherwise there's a chance especially the mega basics end up being a bit too overpowered.
Also, can't you use the same lay-out as the Baby rule? It seems like it's in the right spot and it has the room for an evolution symbol (although I guess it's below the image because it signifies 'evolves into,' so that might be a bit weird). If you're going to allow megas to use the attacks and powers of the card underneath you don't really need the extra space, so the blanks are probably fine as-is in that case. But I'm sure what you're trying to do now will turn out great, so don't let me stop you from trying.
