Firstly, welcome you back! I'm sure much will have changed around the place, though you are sure to see faces you recognise, so make sure you say hello! (That would be my first "to-do" to a newb)
Now, how to start...perhaps point by point? Yes, that seems suitable.
I would not say the game itself is stagnating, more the activity within in. Whilst the game still progresses and has it's loyal followers -- and a few new ones here and there -- the activity of those followers within MD dwindles. Most now stay rooted at the GoE, fighting and grinding. There are still a few occasions where activity picks up, but this is usual due to either chaos (death, slander, ill will, etc) or pre-organised events/happenings (MD Birthday, " Story Makers' " quests, etc).
Working out how to battle this is hard as we would need to determine why people's activity is becoming stagnant. Is it due to laziness? Because the game is turning more towards fighting than RP? Because people simply don't want to RP? Because there aren't enough quests that make others run around like headless chickens? The list is endless.
I (emphasise on I as it's my opinion) believe that the appearance of stagnation is due to less players. There is no longer hundreds of players all playing at the same time, instead there is 30-40. Out of those 30-40 only a handful are "active" enough to run around, do things, make noise...make the place seem alive.
So yes, in short, I believe that there is little we can do at the moment until we start getting more people engrossed in MD. :)
I would suggest (in order):
Speak to everyone - Get to know players, new and old. A lot will have changed in 6 years and you can't expect to know it all yourself. Besides, it's always nice to make new friends!
Get involved - Take part in events and quests, perhaps even make your own. Don't shy away or just choose to read about it later. Some of the best ways (i've found) of making friends and learning new things is by getting involved with everything and anything.
Read, read, read - Read everything. Old forum posts, new forum posts, blogs, adventure logs, creature details, personal papers, announcements...there is a lot of knowledge and help to be found if only people were to read a little more than what's right in front of them.
And finally
Much of what I wrote above applies to this, I feel. Read, read, read, speak to everyone and get involved. From doing that you are likely to learn everything you need. I know that it worked well enough for me, and I was starting off as a brand new player, with no experience.
Though, some of it will come down to your own personality and perseverance, of course. :)