I love the concept of showdown, but there are a couple of things that aren't even gameplay related that can have a negative impact on a run that I think can be easily addressed
No pitcher only rounds
This one seems obvious. Every time I start a draft, it seems like the first silver round is guaranteed to be a pitcher only round, which is where I (And I assume most others) take their bullpen arm. But sometimes the gold round is pitcher only, and it may be possible to have a pitcher only diamond round(in certain divisions) that can make you feel like you just wasted a thousand stubs. Sometimes that pitcher only round has 4 starters, which feels just as bad because instead of getting that Trevor May, Jose LeClerc, or Hector Neris you're getting a Rich Hill or Zach Wheeler. You should guarantee at least 1-2 position players, per round
No duplicate players
During my runs, I've seen countless times where the diamond round would have a really good flashback card (FoTF Gallo for example), only to have the gold round be 3 pitchers and LS Gallo, which again feels like you're getting a raw deal. Similarly you get a good prospect card, then in the bronze round you get that prospect's LS card. Either showdown should let us use both of those Gallo cards, or it should pull the LS Gallo from circulation once the FoTF card is taken. Though it should not work in reverse. getting the LS Gallo should NOT remove the FoTF Gallo from the pool, as it is a clear upgrade.
Add the Gold, Silver, and Bronze Flashback/Legend cards to the pools
Even if just to spice up the draft a little, give us a chance to use Rookie Eckersley or Rookie Andrew McCutchen, or Gold Aaron Judge and so on. there's a lot of fun cards to be had that we just can't use because of a seemingly arbitrary live series designation(When the CPU does not have to follow that rule). This one is less quality of life, and more of an 'added fun' suggestion though.
I figure absolutely none of this will be seen by a developer, but I can try nonetheless. Feel free to post other small suggestions to boost this great potential game mode