Rear Derailleur Question

I have a Altus is a 7-speed MTB-style component group and I need to replace
my rear Rear Derailleur. If I wanted to upgrade it, what could I use without
having to replace the chain or any other part of the drive train?

Thanks

Now now any Shimano is to wide. You could go up to
a Shimano Alivio or LX. That would be reasonable with the pocketbook
and the rest of the group.

Fire up MTB 03