Trend, pattern they are basically the same thing and neither exists.
there is a pattern actually and has been since johto in two ways, team format and evolution. ash always gets the same types on his team:
regional flyer - noctowl, swellow, staraptor
grass starter - bayleef, sceptile, grotle
fire type/starter - cyndaquil, torkoal, monferno
water type - totodile, corphish, buizel
wildcard - heracross, donphan, glalie, gliscor
so team wise there is a pattern...
now evolution wise lets have a look at the above. all the regional flyers have fully evolved and all the grass types have evolved. okay cyndaquil didn't evolve and torkoal can't. none of the water types have evolved
and with the exeption of heracross who can't evolve (he makes up for it with power) all the wildcards have evolved. if you can't see a pattern here i don't know what you think the word pattern means.
this is why i think the writers should evolve buizel and mix things up a bit.