I don't think timebased stacks are the answer, as it would allow some heavy lategame champs with that item(xin, ww...) to savely reach critical mass by just stalling the game, and it would also become a increadible cost-efficient joice for toplaners, wich already have a much higher goldincome than junglers in average.
I think u should gain stacks from kills/assists for the transformation into feralflare aswell to promote interaction.(currently u need 30 big monsters for the transformation and then kills/assists start counting for stacks)
The base stats of wriggles could recive a small nerf to give a clear disadvatage in the early to make up for the late, and it definitly needs a hardcap somewhere, like at 40 stacks, wich still would be 70dmg and 50 heal per hit, wich is pretty damn strong ontop of 15ad+35%as+free longranged ward+gp-10 for 1800g.
The current wriggles/FF definitly needs a chage, if 1 jungler got the lantern and the other not, u either have a stomp early, or the lantern will scale to critical mass so the no-wriggles team got an experation date, the early stomps are a realy poor experience for the other players in the FF team, the experation date is a bad experience for the team against FF(part of the reason why many complain about Yi/Nasus/Jax/Yasou).
If both teams have FF, the team with the better early usually wins, as they will gain mapcontrol and channle alot farm(stacks) into their jungler, while the other jungler needs to help his lanes and is prone to counterjungling, makeing him miss out alot stacks.
With a cap, u could still push early advantages, but the enemy can eventually catch up later if he holds the line as u won't scale into infinity, and with kills/assists giving stacks for wrigles u wouldn't get punished for helping ur mates.