Maps with prevalent line-of-sight blockers can be pretty pesky since only a few towers can counter them and there isn't support that you can provide to make towers immune to them.
I think it'd make more sense for line of sight to be more lenient in Bloons TD Battles 2 compared to Bloons TD 6, since your towers are limited to a total of 3 and maps are generally shorter with Bloons needing to be defended harder.
I think it also leads to maps not being able to have as prevalent of Line of Sight Blockers due to the reliance on specific towers to overcome them. You wouldn't have "Moon Colony" as a map where it's similar to Moon Landing from BTD6 but with a portable lake on it (there's a BTD6 quest like that) because it'd just be too restrictive.
So what if...
Dart Monkey - Spike-O-Pult (partial)
Since Spike-O-Pult lobbing its projectiles over walls entirely would have conflict with its projectile's special power to bounce off of walls for more coverage, the idea is that the spiked ball could be launched over a wall to reach a Bloon but then it would become grounded again
4/10, I'd be somewhat interested in it but it doesn't appeal strongly to me
Sniper Monkey - Night Vision Goggles -> X-Ray Goggles
This monkey tower is popular for global range, but this game's smaller maps compared to Bloons TD 6 make it feel less prevalent in that way, and line of sight blockers are a major element of range restriction.
6/10, I think it'd be neat
Monkey Sub - Advanced Intel
Since it has homing shots and is able to fire outside of its range using intel from other monkeys, it'd make sense for its darts to be able to fly over projectiles. This is also a fairly weak upgrade on the path that gains the ability to ignore line of sight blockers with a targeting option, so it makes sense in that way
5/10, I think it'd be somewhat neat, especially since this upgrade is generally weak, although Monkey Sub DOES already technically have multiple options to attack over line of sight blockers (in this game, Ballistic Missile doesn't need Advanced Intel to attack over walls in the Monkey Sub's range)
Monkey Buccaneer - Crow's Nest
Since the Buccaneer can see from a higher vantage, they can see over line of sight blockers
4/10, as fun as it'd be to play around with, this upgrade is already good. (There's also Aircraft Carrier and Carrier Flagship but to be fair they're not very earlygame-centric so this'd still be fairly different). I'd also like to note for both Sub and Bucc that some maps like Oasis, Ports, and Splashdown (and to a lesser extent dino graveyard) seem to try to balance out water tower strength by having line of sight blockers mess with them (even if on Ports and Splashdown it also messes with non-water towers quite a bit). I wouldn't want both water-only towers to have access to line of sight immunity, and since Crow's Nest is a good upgrade already I would generally rather Advanced Intel have it, even if that buildup is less interesting
Super Monkey - Ultravision
I don't care much for Super Monkey since it just doesn't have a notable earlygame role but I mean this feels right, it's even a Monkey Knowledge in Bloons TD 6. Plus, this is GENERALLY the weaker crosspath in terms of total damage, so it doesn't feel too broken to add, it'd mainly be a buff to the bottom path
1/10, because I have little reason to care for Super Monkey
Ninja Monkey - Counter Espionage
Ninja trickery allows them to attack from behind a line of sight blocker, makes sense to me. Gives Ninja Monkey more of a role, which is nice because this tower feels awkward to me. Also worth noting that this decreases pierce usage reliability as a crosspath for the top path, and costs attack speed as a crosspath for the bottom path, along with not having a strong impact on the middle path
7/10, nice stuff
Druid - Heart of Oak
This tree-centric upgrade gives Druid enough nature power to see and attack past walls, as if they were at a high vantage point in a tree. Also feels right since this upgrade builds into a global, wall-surpassing attack. Druid feels like they're on the weaker side these days, so having this trait could be a nice addition
8/10, solid stuff
Engineer Monkey - Large Service Area
It's best if you can provide service on the other side of a wall using your engineer tactics, especially since as far as I can tell the Engineer can already place inventions on the other sides of walls, they just need to master attacking over them.
3/10, it's OK I guess
Honourable Mentions:
MOAR Glaives could make sense, but also Boomerang is annoying
Bomb Shooter feels like they could have this capability but their Cluster Bomb is actually stronger when its bombs are able to expire early from hitting a line of sight blocker, which I think is fairly neat. MAYBE Really Big Bombs, IDK
Ice Monkey makes some sense, especially since (at least in BTD6, it's harder to test in Battles 2) they will freeze Bloons on the other side of walls if they attack a Bloon that they CAN see. I just don't think that A. Ice Monkey would benefit from it much B. Ice Monkey needs it at all, since they're already such a strong defensive tower. Dare I say, annoying
Glue Gunner's Glue Hose feels very much like it COULD spray its goop over walls, being a hose and all (plus it builds into strike and storm so it DOES just kind of make sense), but Glue Gunner, like Boomerang, is annoying
Dartling Gunner would benefit from it a lot, becoming less unusable on certain maps, but also, and this point will sound familiar, DARTLING GUNNER IS ANNOYING. It's a MERCY that it's essentially not available on every map. Let's MAYBE suggest Buckshot since it builds into towering upgrades and such, but I think that'd feel weird
(That's all. No heroes, none of them stand out to me. A hero or alt who lets monkeys surpass line of sight blockers would be cool eventually. Maybe a Quincy alt who takes more of a support role, helping other monkeys with their ability to aim at Bloons including over walls)
LMK what you think! Wouldn't want this to be power-creep-y (especially with the water towers) but I really do think that line of sight blockers as a mechanic are just too restrictive in this game.