There are no patterns when it comes to evolution...
Grass types
Bulbasaur didn't evolve at all
Chikorita evolved once
Treecko evolved twice
Turtwig will have evolved either once or twice..more than likely once
Either way you slice it, there's no pattern. They wouldn't have had Turtwig evolve if they were going to go the "restart the pattern" route. You could argue that they're starting to count down now, Turtwig will evolve only once and Ash's grass type in the next generation won't evolve at all. The thing is, we don't know what will happen in the fifth generation. You can't claim there's a pattern for something that hasn't even happened yet.
Fire types
Charmander evolved twice
Cyndaquil didn't evolve at all
Torkoal couldn't evolve
Chimchar will either evolve once or twice
Again, no pattern
Water types
- Squitle didn't evolve because Charizard was the most popular evolved Kanto starter and Ash got that instead of Blastoise.
- Nothing evolved in Johto..That's Totodile's excuse. Not even the wildcard which usually do evolve, Phanphy.
- Corphish was comic relief and that wouldn't work if it were a Crawdaunt
- If Buizel doesn't evolve it's because it's more popular than it's evolved form and the other pokemon were more in need of evolution. Buizel was a promotional pokemon for DP, unlike Floatzel
Now while there may be a pattern at first sight if you really think about it you'll quickly see that that is just not true.
Why would the writers limit themselves by setting up ridiculous patterns?
Okay, scenerio. There is a widely popular water starter in the 5th generation. It's not girly so they decide to give it to Ash. The thing is, it's evolved form is even MORE popular. It's evolved form is as popular as Charizard was in Kanto. Can you really see the writers sitting around having this conversation? I can't
Writer: Great news! The new water starter's evolution, Hydrosaurus, is very popular! It's selling tons of plushies already!
Director: Oh well that's nice...
Writer: Yeah, so I was thinking..maybe we could evolve Ash's Hyrdotard into Hydrosaurus. Promoting it in the anime would make it even more popular
and more money for us, right?
Director: Blasphemy! We have a pattern to uphold!
Writer: But..
Director: I don't want to hear it! We've never evolved water pokemon and it's not changing how!
Writer: I thought the reason we didn't evolve the past water pokemon was for plot reasons
Director: Nope..it's just easier with the patterns..