Being present in every european language (and in fact, in about every languagge known to science), the adjective can be regarded as an important element in any language should it be useful.
It's form is diverse in the languages of europe, making it almost arbitrary, which way to mark it. As established in norming the verbs and plural, a suffix would suit this issue.
Former incarnations of the language have suggested a range of adjective andings, with the latest two probably the best-working.
Words ending in a vowel become an adjective by adding an -n at the end, as in europan, derived from europa with an n added.
In order to increase variety within the language (thus improving the way the language sounds), the suffix for words ending with a consonant shall be completely independent of the other one.
There is one adjective suffix that is quite common throughout the europan languages, being an i followed by a consonant, linguistically speaking iC.
Using the most common form of this suffix, -ik would seem useful and shall thus be the suffix of choice. Thus, to make a word that ends with a consonant into an adjective, add -ik. For example, the adjective of internet would be internetik
In some european languages (like German), adverbs are often indistinguishable from adjectives, it would thus seem to be no problem to let them use the exact same form.
The adjective may also replace the genitiv case that is used in many european languages to signify ownership. Thus, for example, Anna's would be Annan, Tom's would be Tomik etc.
Austro-European opinion(by Amatio) Edit
In Latvian(adding Lithuanian as it familiy language brother) mostly men-adjectives end up with -s(Lithuanian -as) and women-adjectives end up with -a. In Russian(not included in Europan, but reflects on many Slavic languages) There is 3 divides to adjectives - men, womens and middle. Each up ends with different letter and sometimes without ending-letter. Men-adjec. end up with with nothing, while women end up with -a. Middle-adjec. end up with -e. Of course these are most cases. So, if for basic taken is -n and -ik, than I would prefer to use -a in women adjectives, not braking -ik as men adjective. So there would be Anna and Tomik. Lila(Lilly) and Jonik(John).
Genative we can make easy by adding for example -u to adjective. Like u Lila, or u Jonik. Or any letter that suit elocution. Futer examples: o Jonik, en Jonik, lu Jonik, an Jonik. Last one is also great choise.