If theres gonna be a pseudo grass type, than the grass type starters will be even more overlook.
And right now there are alot of great non-legendary grass type, while not as much as succesful ice-type.
The only solution for grass type starter to be succesful is to make them more look cooler, which is hard.
There are plenty of popular and successful Ice-types. Walrein, Beartic, Dewgong, Cloyster, Weavile, Glaceon, Froslass, Glalie, and Mamoswine, just to name a few, are all Pokémon that, when handled correctly, can be very effective in battle. Some of these Ice-types also have dual types and abilities that balance out some of Ice's weaknesses. For Walrein, for example, being a Water-type neutralizes the effect it would receive from Fire-type attacks. Also, with Thick Fat, it gains even more resistance to Fire-type attacks. Again, Froslass is made immune to Fighting-type attacks due to its secondary Ghost-type.
Also Ice-type, strategically speaking, is more advantageous than Grass-type. Ice is weak to just as many types as it is strong against. Its weak to four types–Fire, Fighting, Rock, and Steel–while also being strong against four types–Grass, Ground, Flying, and Dragon. Grass on the other hand is only strong against three types–Water, Ground, and Rock (and a resistance to Electric)–while being weak against five types–Fire, Flying, Ice, Bug, and Poison.
Also, a Grass-type Pseudo Legendary wouldn't necessarily detract from the Grass starter. That really depends on a number of factors: aesthetics, availability, etc. And again, I wasn't necessarily talking about the Grass starters, but the Grass-type in general. A bunch of the Grass starters already do look cool and can be useful in battle. Meganium, Sceptile, Torterra, and Serperior all had really cool, unique looks. Grass, as a type, however, gets bad PR and has too many weaknesses compared to the other starter types. What they need to do is either make some very high-stat Grass Pokémon that aren't Legendaries, such as a Pseudo-legendary, or retcon the Grass type to not have as many weaknesses. I honestly don't even get Grass' weakness to Flying-types. I mean, when was the last time you saw a bird go toe-to-toe with a plant? If anything, birds and flying creatures depend on plants for their survival. They make their homes in trees and other types of plants and depend on plants as a source of food in many scenarios.
