What's next refers to,what to buy next after getting the one you've always wanted,or
What's next refers to,after your enthusiasm for watches dies down?
To both these questions I think it's personally and only you can decide your next move.
As for weather is buying a watch an investment?,if you're an investor,yes. Obviously an investor knows what to buy.
And if you're like the majority,buying a watch is definitely not an investment.And if you made some money along the way,just count yourself lucky.
But you should not let the thought of buying a "no resale value" watch deter from buying it,otherwise your collections will only be Rolex,Rolex and more Rolex.