Before DP there was no urgent need to consider if Brock should leave or not. DP was the turning point showing that Brock can't interact will with every new character. I'm certain the writers wanted a fresh new vibe with BW but I seriously doubt they forgot how much Brock's character actually dropped at this point because in reality they could have kept Brock if they really wanted to, Hoenn was almost as much of a fresh start as BW but Brock came back anyway so the same could have applied here.
Hoenn and Sinnoh were in a way treated as "new start"but difference is that BW is portrayed as more of a fresh start than ever before with no references to past and older characters being made,contests being dropped,Unova being portrayed as far and isolated region not containing older pokemon etc.
Like i mentioned earlier how well some character will get along with new characters depends on huge part on how much they are going to have in common or have something through which they could easier relate.
In original series reason why Brock and Misty had good chemistry was because they were both gym leaders having some things in common as well sharing same views about some things like romance complimenting each other both through differences and similarities.
With AG group Brock had Max with who he had things in common both having big knowledge about pokemon helping Ash in pokemon battles with their tips.Max also admired Brocks skills as breeder and overall knowledge being easier for their characters to relate to each other.
Hoenn not being focused so much on expansion of main girl story being treated like almost equal to Ash like it was case with DP heroine also helped Brock to have time to interact more with May resulting in having more memorable moments than i can say for Dawn.
Main problem with Sinnoh was that show was too much focused on development of Ash and Dawn stories and their portrayal as equals which resulted in Brock often being left behind.He had some moments in Sinnoh with them but not as much like i can say for previous groups.
In Unova it depends on character if Brock will have some things in common which could allow for good dynamic,This of course isnt necessary but it certainly helps.
However change of formula to only on main star would certainly go in his favor since there would be more available screen time for writers to distribute among characters with Brock having better chance to establish some memorable relationship with others.
Yes I know people get excited when old characters return. But let me ask you this, do you think Brock would have got the same reaction if he was confirmed to stay in BW right after DP, that say May or to humor you Misty got for their cameos? The M girls got major fanfare and hype for their cameos, can you honestly say Brock would get the same if he had never left from DP to BW? I should make a poll about that.
If Brock returned right away for Unova reactions would be pretty divided and same like they were for DP more or less with some liking that he wasnt replaced while others complaining how "Brock is stale,horrible"etc,etc,etc.
If Brock returned right away people wouldnt get too much excited
butif he was absent for longer period of time like its case with some other characters you can bet how reactions from people would be vastly different being much more interested in Brocks return than they would be now.Which is normal since long gone characters are missed by people.
With May and Misty granted there always was present more of a hype and if they appeared or returned to main cast(especially those who were gone for years like Misty)excitement would be probably bigger than its case for Brock but i suppose how fact of having stronger following is responsible for this too.
