I know some of these have been suggested already, but I think all of the forme names in the
GP standards should be added as aliases (excluding ones that wouldn't make sense for Showdown, like Terapagos-Terastal being called Terapagos). A few already work in some places thanks to fuzzy searching, but not everywhere (e.g., the /analysis command) supports that.
Currently missing:
Basculegion-M (Basculegion)
Basculin-W (Basculin-White-Striped)
Calyrex-I (Calyrex-Shadow)
Calyrex-S (Calyrex-Ice)
Galarian Darminitan-Z (Darmanitan-Galar-Zen)
Dialga-O (Dialga-Origin)
Indeedee-M (Indeedee)
Ogerpon-C (Ogerpon-Cornerstone)
Ogerpon-H (Ogerpon-Hearthflame)
Ogerpon-W (Ogerpon-Wellspring)
Paldean Tauros (Tauros-Paldea-Combat)
Paldean Tauros-F (Tauros-Paldea-Blaze)
Paldean Tauros-W (Tauros-Paldea-Aqua)
Palafin-H (Palafin-Hero)
Palkia-O (Palkia-Origin)
Tatsugiri-C (Tatsugiri)
Tatsugiri-D (Tatsugiri-Droopy)
Tatsugiri-S (Tatsugiri-Stretchy)
Terapagos-S (Terapagos-Stellar)
Ursaluna-B (Ursaluna-Bloodmoon)
Urshifu-S (Urshifu)
Urshifu-R (Urshifu-Rapid-Strike)
Venomicon-E (Venomicon-Epilogue)
These two aren't actually in the standards, but I think having them would make sense
Basculin-B (Basculin-Blue-Striped)
Terapagos-T (Terapagos-Terastal)