Swype keyboard used to have this feature built in. Put the cursor on a word, any word (made earlier by swype's edit keyboard, which had cursor controls), and tap the Swype key. You'd be presented with a list of alternates/stems, with the current word as the root. At minimum you got simple plurals

Gboard has a similar edit keyboard, although I find it's a tad harder to get to than Swype's