Why would bonfires on Lag Baomer have anything to do with Shavuos? Is there fundamentally something wrong with celebrating something differently on different days? On Friday night, we say “zecher litziyas Mitzrayim” as part of kiddush, so by that token, should we eat matzah and maror every Shabbos?
Your analogy is not valid.
We were commanded by God to eat matzah and marror on Pesach, while we were not commanded to do so on Shabbos.
OTOH, building bonfires is not a command from God, but something that we, ourselves originated. As such, if we can do so for Lag B’Omer, I don’t see a reason why we can’t do so for Shavuous as well.
The Wolf