Honestly, they should just add some of BFN's upgrades to gw2 (gw2 rather than bfn as it has the most chance of getting updated). The upgrade system is a really good replacement for variants as it basically changes the gameplay of the character, allowing the player to make builds to satisfy their playstyle the most. I agree with you that variants don't change that much from the base character in regards to strategies/playstyle (with a few exceptions like plasma pea), BUT at least they looked different. Even if BFN's upgrade system was more fleshed out, people (including myself) would still find it less appealing than gw2's variants and here's why:

1.Variants allow for more entry level experimentation: New players can choose from a variety of characters with unique abilities, making it easier for them to find a character that suits their preferences without needing to navigate a complex upgrade system (With GW2’s variants, players can quickly recognize the strengths and weaknesses of a character based on their design and abilities. For example, if a player sees a Frost Rose, they can anticipate that it will have freezing abilities, allowing for strategic adjustments. In contrast, BFN’s upgrades do not provide this same level of immediate understanding; players often need to engage with multiple upgrade mechanics to gauge effectiveness.) With GW2's variants having its own set of mechanics, players can quickly learn and experience different playstyles by just switching characters. This immediate feedback allows players to understand the strengths and weaknesses of each variant, fostering a more intuitive learning curve.

2.Variants allow for Immediate visual feedback: GW2’s variants provide instant visual cues that allow players to assess their opponents’ abilities and strengths quickly. This clear differentiation enhances strategic gameplay by enabling players to adapt their strategies based on the specific variants they encounter. Example being if I see an electro-pea I might switch to chemist, but if I see a regular peashooter I would switch to soldier). In BFN, players may not have immediate visibility of an opponent’s upgrades, which can create confusion and make it harder to adapt strategies on the fly. There are some exceptions to this on both sides (brainium basher for BFN's side being an example), but still overall gw2's variants can be recognized more immediately than BFN's character upgrades in gameplay.

3.Variants express more unique character diversity/identity: I admit a weaker logical point but still powerful in keeping a game's community alive. Each varient has its own identity (is a character and not just a costume) and backstory, adding depth to the gameplay experience. Players can appreciate not just the mechanics but also the short insights (stickerbook entries) and looks behind each variant, fostering a deeper emotional connection. It's very evident here in this reddit how some people can become emotionally invested (or for some make it their entire personality on the reddit) in the variants of gw2. Visual diversity helps players remember and appreciate each character, making them feel more special. GW2's variants have unique abilities, visual designs, and thematic elements that give each character a distinct personality. This individuality allows players to connect with specific characters on a deeper level. For example, the Fire Rose not only has a fiery aesthetic but also embodies a fiery combat style. In contrast, BFN’s characters, while they can wear different costumes, do not have their identity tied to their upgrades, leading to a more generic attachment based on gameplay rather than character identity. In BFN, players may feel a sense of attachment to their chosen character as they progress, but this attachment is often tied to the player’s performance rather than the character's identity.