The last SEC to be revealed for BT-09 may be the next most popular card.
The black/purple LV7 Dexmon has what it takes to become the next anti-meta card.
His first effect reduces his play cost by -3 for each Digimon and tamer that your opponent has, a very straightforward effect that punishes tamer-centric decks such as the Zoe hybrid and the Tommy hybrid, which are easily the current tier-1s. To put this into perspective, Dexmon would cost only 2 to play when your opponent has something like 4 tamers and 2 Digimons.
His second effect not only de-digivolves all of your opponent's Digimons by 1 level, but also deletes all that is LV4 and lower. In other words, playing 2 Dexmons consecutively in a turn will clear the opponent's board of LV6 Digimons, assuming they digivolved normally in a LV4-LV5-LV6 sequence (again, punishing the current Zoe and Tommy hybrids). This effect is also highly effective against Armor Digimons.
His third effect punishes the opponent when he is not taken down during their turn, by deleting all of your opponent's Digimons with the lowest play cost. In other words, your opponent is forced to play some sacrificial Digimon before the end of their turn in order to protect their better Digimons, and this becomes more obnoxious the more Dexmons you have in play.
Dexmon's weaknesses though, are also pretty obvious.
Firstly, his "de-digivolving then delete" effect would be less effective against X-Antibody decks that may have a larger stack of LV5s, making them a little bit harder to delete. It is also not effective against big Digimons that do not have digivolution cards, the usual suspects such as Beelstarmon, Lucemon Chaos, and Omegamon Zwart Defeat.
Secondly, his overall effectiveness can be thwarted just by knowing that he is included into the deck. The opponent will avoid playing a large pool of tamers and Digimons, or avoid ending their turn with a LV5 Digimon that can be de-digivolved into a LV4.
However, these weaknesses can be circumvented.
Dexmon in the Dex archetype
A no-brainer. Being of the same archetype, Dexmon benefits from the synergetic engine.
[BT9-081] DexDorugoramon has an [On Deletion] effect that can play Dexmon from trash without cost, which is easily the most amazing callback effect. No other Digimon can play another LV7 Digimon for free, from trash.
With the LV4 and LV5 Dex Digimons providing the much needed milling, in addition to the strong board control from the original Dorugoramon line as well as DexDorugoramon, this deck should prove formidable and popular in the coming meta.
Both Dorugoramon and DexDorugoramon excels in controlling a smaller board of 2 or 3 Digimons. Dorugoramon could easily eliminate up to 2 medium-sized Digimon, something around a play cost of 6-7, which helps to setup DexDorugoramon's [When Digivolving] effect against the bigger Digimon. Dexmon would be perfect here, offering robustness against wider boards instead.
Meanwhile in a purple alternative, the new [BT9-079] GrandDracmon should also work well with the Dex archetype, since they are mostly [Undead] Digimons. GrandDracmon can accelerate digivolution of the Dex line thanks to it's [End of Attack] effect, which could hopefully help to launch DexDorugoramon more efficiently.
Dexmon in Security Control
As it happens, Dexmon does not really need to be in a specific archetype.
While we do yet not know if the yellow security control deck would survive the coming meta, assuming it does, can be strengthened by Dexmon.
Dexmon can digivolve from [BT8-082] Ophanimon Falldown Mode, a staple of most yellow security controls. While more expensive (in digivolution) than the popular Zwart Defeat and less effective in security, Dexmon can deal more damage on-play and when digivolving, (conditionally) plays cheaper from hand, and has an immense presence when he remains in play.
The defensiveness of a security control deck may work in favour of Dexmon, punishing an opponent with a wider board in his mid/late game.
Dexmon with Beelstarmon
Another easy engine since he can digivolve from [BT6-112] Beelstarmon, replacing Zwart Defeat as the preferred choice of LV7.
Dexmon extends Beelstarmon's damage range, which is usually limited to single target 7-cost purple option cards. While many Beelstarmon decks play copies of [BT6-095] Happy Bullet Showering and some [BT6-105] GewaltSchwarmer for wide-range deletion, they are often too conditional and does not delete what is intended. With Dexmon, the deletion range is not only extended beyond DP-based and cost-based targets, but also improves the deck's robustness since pure-deletion effects are less effective against Armors and the Greymon X-Antibody line.
Dexmon with Mugendramon
With [EX1-073] Mugendramon expected to a little more popular in BT-09, it is fascinating to know that Dexmon can also work in this deck.
While Mugendramon is a very defensive card, his engine is often weak towards a wide board (and no Chaosdramon is often not a solution). Mugendramon decks are slow, and playing a Mugendramon amid an opponent's wide board of Digimons would usually offer little to no protection since he can only block once (without Tai Kamiya).
We might find solace in Dexmon during these sticky situations, first reducing play cost by taking advantage of the opponent's wide board, and then reducing that board enough for Mugendramon proper.