I am going to start this off just diving right into the subject.
We as players, professors, and just fans of such a beautiful and long lasting franchise, one that began aimed at children that has now reached and extended itself over to intergenerational, do desire a switch and change of pace every so often. If it were in my hands I would choose at random 10 Pokemon that we as a team had in a pool and said, "These have not been chosen as a tcg card for a while. Let us select these ones for this expansion, or at least work them into several sets over the next year."
There are plenty of cards we have not seen as of lately, for years in that matter. Pokebeach itself mentioned Kabutops. Yes, it most definitely is one of the more popular and sought after fossil Pokemon due to its place in the first generation, but also the design has been appealing to users for quite some time. This is just an example, but hear me out on what I would truly recommend as I might impress you, or at least try to sound appealing myself.
The Pokemon I am about to name is not one I will be mentioning due to the high popularity and praise it has garnered over the years, especially the meme that followed and made it become an incredible superstar. This was my personal favorite Pokemon of the Kanto Region. Whenever as a child someone would ask me what my favorite Pokemon are if I could have them in real life I always responded, "Porygon" but there was one other. Even if I had to search my entire life to have it I strongly desired an Omanyte.
Yes, Lord Helix itself is quite the star in the recent years due to the meme that made it famous. I believe an Omastar-GX would prove to be quite popular, even if the card itself is not even useful.
Why would I want a card to come into existence if for the fact it could be unplayable due to the Restored Pokemon mechanic? Omastar-GX would sell, especially as a promotional Premium Collection card, due to the fact it is Lord Helix fully visualized. There would be plenty to own one just to be a part of the meme fully realized in person.
Fossil Pokemon are already very popular in their own right, but Omastar-GX has a lot going for it even if it is not real. There is a specific Omastar-GX fan card going around right now in posts as responses to those hoping to have their packs blessed and open good cards. I believe pulling in the fans of Omastar and Omanyte, the fans of the meme, the fans that would love to have fun and participate to allow it to grow even further would buy the Premium Collection Box it would come in by the cases.
How would this appeal to children? Well, there are plenty of children that already have been impressed and recently engaged with Omanyte and Omastar due to the release of Pokemon GO. There are also children that love the fossil Pokemon, and those that have parents who have already exposed them to the Lord Helix meme. Omastar is just a great Pokemon in general and brings a lot to the table in "sellability".
I understand that this may be extensive, but it does cover quite a few of the most important points of creating Omastar-GX. In ending I believe this would be a product that could be moved quickly and become a part of history.