...or if I just want to showcase my ideas for existing companies with not-so-great logos, can I put that in my portfolio? or even fictitious companies, is that okay to build up on portfolio?
I see no reason why not.
The entire world is not so contract-obsessed as the US, so unless you have a massively complicated contract that mentions this (and that to me would be wildly bizarre), go ahead.
You should also be able to put the logos that are in use in your portfolio too. That is kinda what a portfolio is.
And fictional logos? Absolutely. It is nice to see some stuff that are entirely from your own imagination too.
Well, I did couple of successful master classes about portfolio in past and would suggest - use any your work in portfolio that show your skills on the edge. No any excuses for mediocre work - "it is approved by client". It is YOUR portfolio, if you disagree with client, use what you think is best.
Client must sell own services/products. You must sell own.