[[boa#Finnish|boa]]

The Finnish oblique plural marker is perhaps best considered //j// rather than //i// (with vocalization rules to produce /i/ when not intervocalic), if you wanted to analyze it in terms of a single agglutinative marker.

However, you could just as well analyze Finnish plural stems as rather having two markers - an agglutinative ending /i/, and a vowel mutation rule that changes /ä/ → /i/, /a/ → either /o/ or /i/, and /i/ → /e/. In the kala and risti types they co-occur, with the result that e.g. //kalo-i-a// produces kaloja; but elsewhere you only ever have one or the other. There's no real benefit to an analysis that e.g. vesiä would be underlyingly //vesi-i-ä//.

(As we see from words like punainen or ajaja, the rule *aj > *i/*oi had ceased to be surface-phonological already in Proto-Finnic.)