This is my take at building a "U.A." deck. The "U.A." archetype is based on various pop culture references regarding "Future Sports", mixing a solid amount of already existing sports in one giant mash-up (even including race cars since some of the newer "U.A." support also counts as "F.A."). During the game, they are played as a Hybrid deck, being a mix of both beatdown and aggro strategies while also allowing field control via negates when it counts. To allow such a smooth transition, all "U.A." monsters (except one) can Special summon themselves by returning an "U.A." monster to the hand, giving you the option of attacking normally during your Battle Phase only to switch your monster line-up during Main Phase 2 to have disruption ready during the opposing turn.
The monsters are almost entirely from the "U.A." archetype, except for the three copies of "Ash Blossom & Joyous Spring", who I run for the obvious negation via handtrap; "U.A.s" have a good amount of consistency boosters and do not really lose any tempo by putting a few handtraps in, so run whatever option you either deem is the best or, alternatively, the most budget-friendly. The "U.A." core consists of three copies of "U.A. Midfielder" and "U.A. Libero Spiker", who both have solid switch effects to be able to even switch out monsters during the opposing turn, but I cannot stress enough that they are also there to provide non-tribute Normal Summons, something which the archetype was and to a certain extend still has problems with. Without a monster on board, you cannot start the switching schenanigans, which will leave you with quite a few bricks in form of monster cards. Talking about switching, the options you can go into are two "U.A. Perfect Ace" who negates a card for discarding one from your hand, a copy of "U.A. Blockbacker" who can switch the position and then negate the effect of a Special summoned monster, a copy of "U.A. Mighty Slugger" who works like a "U.A." version of an "Ancient Gear" monster and therefore prohibits the opponent from activating stuff during his attack and two copies of "U.A. Dreadnought Dunker" sporting both piercing damage and destroying a card whenever he is able to inflict damage to the opponent.
The last monster in the deck is "U.A. Playing Manager", played at three copies (although you could lower the ratio to two if necessary). This card is a beast, because he can Special summon himself when a switch is happening, giving you some swarming and an additional beater. But whenever he is Special summoned, he can decide whether he wants to destroy a card on the field or negate the effects of all non-"U.A." monsters, which is even better. He is easy to recycle, since you can simply bounce him to your hand as cost, Special summon another "U.A." monster, then bring him back in because his summon effect triggers, giving you another activation of his effects if he has no use them during that turn. And with the help of the Level 4 monsters, you can even switch him out and bring him back in during the opposing turn, even giving you another disruptive option.
The Spell/Trap line-up has various staples that you will find in other "U.A." decklists aswell; simply because those cards are that useful. Both "U.A. Hyper Stadium" and "U.A. Stadium" are played at three copies, giving you a solid amount of helpful effects which could look something like this: Play "U.A. Hyper Stadium", search a Level 4 monster via its effect, use the second effect to reveal a "U.A. Stadium" in your hand to gain an additional Normal Summon for the turn, play "U.A. Stadium" from your hand, Normal summon the searched Level 4 monster giving you another search via the first effect of "U.A. Stadium", play another Level 4 monster via Normal summon (either because it was already in your hand or because you searched it with "U.A. Stadium's" effect) to gain another search into a high-level monster and continue from there. The Field Spells are crazy useful and the ratio works fine in my mind. I even run one copy of "Terraforming" to get the missing piece more often. The three copies of "Foolish Burial Goods" are mostly for ditching "U.A. Penalty Box", which can search a "U.A." Spell card when banished from the graveyard, giving you yet another searcher for the Field Spells or for the single copy of "U.A. Powered Jersey", which in combination with "U.A. Dreadnought Dunker" can end games rather consistently. "U.A. Signing Deal" is both a playstarter, since it puts a monster on the field, as well as a search option because you can easily bounce the card and then resummon it with its effects activatable; the single copy of "Reasoning" serves a similar purpose. The deck works very well without touching the Extra Deck, therefore the maximum amount of "Pot of Extravagance" serve as very solid draw options. The generic rest is one "Reinforcement of the Army" to search for the Level 4 guys and therefore get them into my hand more consistently, which allows different searches via the Field Spell line-up and one "Twin Twisters" against opposing backrow. One argument that I have encountered against "Twin Twisters" is that you always have to discard a card, which can be pretty damaging to "U.A." but I do like to have the option of getting rid of a potential copy of "U.A. Penalty Box" in my hand; but feel free to run a different backrow removal.
The Extra Deck, as is often the case, mostly features options for various situations in the game. "Barricadeborg Blocker" makes Field Spells reuseable, "Missus Radiant" can be used to push a little harder since the entire "U.A." line-up is Earth Attribute, the "Knightmares" are well-known and provide various removal effects, "Isolde, Two Tales of the Noble Knights" can be used to search but will often not be necessary, "Abyss Dweller" is a counter option against graveyard-heavy decks, "Evilswarm Exciton Knight" can serve as a field reset, "Castel, the Skyblaster Musketeer" brings non-destruction removal, "Tornado Dragon" is another anti-backrow option, "Number 61: Volcasaurus" can sometimes be used to burn for game, "Number 11: Big Eye" can steal monsters from the opponent which clears the field of one monster to get over while also providing you with another attacker, "Mecha Phantom Beast Dracossack" provides tokens and further removal and "Number 38: Hope Harbinger Dragon Titanic Galaxy" will bring Spell negation. Similar to the "B.E.S." deck I posted a while ago, "U.A.s" are not really in need of summoning any of those cards, but it is nice to have the option instead of leaving the Extra Deck empty (which would be a bad choice anyway due to "Pot of Extravagance").
There are two deckbuilding details I want to mention still: Some decks run as little of the "U.A." monsters as possible, potentially running only one copy of every high-level monster because they are easily searchable and accessable. That is a valid strategy and can be played as such, I simply wanted to have the option to still have some of my playmakers after losing one to destruction effects or being forced to discard them. Pick whatever approach seems best in your mind. Number two is about the Extra Deck: Since "Pot of Extravagance" banishes six cards, there are decks that run about five Extra Deck monsters at maximum copy count to increase the chance of still having copies of everything after banishing. This is also a solid approach to take, the only thing you need to ask yourself in order to find out what is best in your deck is whether you like a plethora of options pre-"Pot of Extravagance" or if you want access to all of the card post-"Pot of Extravagance". Those are only some pointers I found noteworthy when building the deck.
"U.A." is certainly a quite unique and powerful deck to play. The option of turning an Aggro-board into a disruptive wall is both strong in-game and fun to pull off. If you are interested in the rest of the "U.A." archetype and want to read my opinion on all things "U.A.", you can visit my blog, where I recently posted a "U.A." archetype analysis, here: https://cubiccreativity.wordpress.com/2020/10/20/archetype-analysis-u-a/