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One thing to consider is how does the topper secure to the table, if at all? Mine has a set of rubber legs at each end that sits within the well above the legs which keeps it from moving around when in place.
This also leads to my second point, but I think this is more in the carpenter: tolerancing. Wood moves and settles - our rubber lugs currently don't make into those recesses any longer because the wood has settled.
My table has a small groove cut into the top of the 'armrest' area when the topper is off. This allows you to hold cards without actually holding them! Might be worth a consideration for you.
Finally, consider if you want the table in an oil finish, or varnished. What colour stain would you like on it? Is the vaulted area going to have a felt bottom? If so, what colour would that be?
I don't have a table, but I've heard that the 'armrest' width needs to be of sufficient size, so as to not cut off blood flow to the hands. Around 10cm minimum.
Also consider having some sort of hidden mini-table for each player on each of the 4 sides. Something like a circular swing-out table from underneath the main table, for items out of view of other players.