You could plant another plum or something else...but it will take some ground preparation first.
From the 'gap' I gather there is more of fruit trees nearby?
Your old tree has used up lot of goodness from the ground and nearby neighbours will give competition too, so you need to dig in plenty of 'good stuff' first to give any new comer a fair chance...even adding some more soil from else where. I would also use some those 'fungi granules' (sorry..I've just woke up and brain is still mush...can't think the proper name for them...rootgrow or something) to get the tree roots establish that little bit better.
Oh..just noticed you said..40 yrs old tree...hmm...could you have 'gap' year this summer..plant something totally different there for this year(flowers/veg?)...continue making the ground more 'lush' again and get back planning what to plant there later on the year.
It will give you chance to see how the neighbouring trees will react for change of space.
Good morning..