Which fruit trees

I have space on my allotment for two more fruit trees to go with my existing ones.  I need ones which can be espaliered.  I have four apples already and wanted a different type of fruit to espalier.  Not plums or pears, as I have a source of plenty for them.  Any ideas?

Like Yorkie says you could try cherries, but of all my trained fruit trees the cherries seem least happy & productive grown like this (cordons in my case).  I have a (grafted) redcurrant cordon that does very well, or you could use the space for a blackberry and train the canes.

Thornless blackberry - excellant idea.  thank you.