Im going to go over them mechanically and not about what parts of cultures they represent or are inspired by
I get that TappedOut has a very active custom card community - that community is probably the most active on the site - but you have to think about the limitations of the page you are using
Generally, the whole exercise would be quite simple, but there are three pitfalls: Some sequences of 3 are repeated on the sheet (there are three such instances on the Gym Heroes commons sheet)
These will transform after you fulfill some kind of condition on the card, usually either an activated ability (i.e., paying to exile Terra, Magical Adept) or triggered ability (i.e., having six permanents in the yard for Exdeath, Void Warlock at end of turn)
Auction Business will take all reasonable precautions to protect items held in storage pending shipments but provides no insurance coverage for that purpose and bidder's agree the auction business stores items completely "at risk" to the bidders and/or winning buyer