Any use for meganium?

Depends on its cp. If it is under 1400 cp it is a good prestigier, but if it is above 1400 it is just a worse venusaur.

That's a A ranked moveset, but at this point Meganium is probably a "bad" venusaur/exeggutor, depending on the CP range you can have it as an optión to attack/prestiege into Rhydon/Vaporeon

I use mine to prestige gyms when I'm useing my lanturn for bubblestrat but mine is also a low cp but it could be good for attacking gyms but since you seem to have a pretty decent grass army it's not necessary

It's pretty much on equal footing with Tangela if it has vine whip/solar beam.

The only real niche for Meganium (and Tangela, Bellosom) is the pure grass typing: It will be a better choice than the other dual typing grasses (Venusaur, Vileplume, Exegg) when there's a situation where the other typing cripples them (e.g. Psychics cripple venusaur, meganium resists ground when venusaur doesn't) but in all other cases, venusaur will prove to be better. Same story for exegg vs bug (double weak vs single weak) and exegg vs dark... you get the picture

Do bear in mind, Tangela has a next evolution in Tangrowth (can't remember if Gen 3 or 4) which is a beast!! but that's another topic of discussion for later.

" is it just a nice trophy or does it have some use somewhere"

If you get a good moveset, it's one of the best prestigers in the game against the 2 most common gym defenders in the my area -- Rhydon and Vaporeon. My CP ~1400 VW/SB Meganium sees a lot of gym battle time (for training up gyms), among my top 5 in terms of how often I choose it for battles.

Meg's backside looks better than Venu's or Egg's when attacking, at least to me :D And double typing sometimes is annoying, Venusaur gets hit harder with Psychic moves.